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Markdown
283 lines
15 KiB
Markdown
---
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category: tool
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tool: Docker
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filename: docker.bat
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contributors:
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- ["Ruslan López", "http://javapro.org/"]
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- ["Michael Chen", "https://github.com/ML-Chen"]
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- ["Akshita Dixit", "https://github.com/akshitadixit"]
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- ["Marcel Ribeiro-Dantas", "https://github.com/mribeirodantas"]
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---
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Docker is a tool that helps you build, test, ship and run applications
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seamlessly across various machines. It replicates the environment our software
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needs on any machine. You can get Docker for your machine from
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[docs.docker.com/get-docker/](https://docs.docker.com/get-docker/)
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It has grown in popularity over the last decade due to being lightweight and
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fast as compared to virtual-machines that are bulky and slow. Unlike VMs, docker
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does not need a full blown OS of its own to be loaded to start and does not
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compete for resources other than what the application it is running will use.
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VMs on the other hand are pretty resource intensive on our processors, disks and
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memory hence running multiple VMs for various applications becomes a challenge
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in a limited capacity architecture.
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<pre>
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┌────────────────────────┐ ┌───────────────────────┐
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│ ┌───────────┐ │ │ ┌───────────┐ │
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│ │ App │ │ │ │ App │ │
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│ └───────────┘ │ │ └───────────┘ │
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│ ┌────────┐ ┌────────┐ │ │ ┌────────┐ ┌───────┐ │
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│ │ Libs │ │ Deps │ │ │ │ Libs │ │ Deps │ │
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│ └────────┘ └────────┘ │ │ └────────┘ └───────┘ │
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│ ┌───────────────────┐ │ │ ┌──────────────────┐ │
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│ │ Guest OS │ │ │ │ Guest OS │ │
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│ └───────────────────┘ │ │ └──────────────────┘ │
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│ VM1 │ │ VM2 │
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└────────────────────────┘ └───────────────────────┘
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┌──────────────────────────────────────────────────┐
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│ Hypervisor │
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└──────────────────────────────────────────────────┘
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┌──────────────────────────────────────────────────┐
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│ Host OS │
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└──────────────────────────────────────────────────┘
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┌──────────────────────────────────────────────────┐
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│ Hardware Infrastructure │
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└──────────────────────────────────────────────────┘
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(VM based architecture)
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┌────────────────────────┐ ┌───────────────────────┐
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│ ┌───────────┐ │ │ ┌───────────┐ │
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│ │ App │ │ │ │ App │ │
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│ └───────────┘ │ │ └───────────┘ │
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│ ┌────────┐ ┌────────┐ │ │ ┌────────┐ ┌───────┐ │
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│ │ Libs │ │ Deps │ │ │ │ Libs │ │ Deps │ │
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│ └────────┘ └────────┘ │ │ └────────┘ └───────┘ │
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│ Container1 │ │ Container2 │
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└────────────────────────┘ └───────────────────────┘
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┌──────────────────────────────────────────────────┐
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│ Docker │
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└──────────────────────────────────────────────────┘
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┌──────────────────────────────────────────────────┐
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│ OS │
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└──────────────────────────────────────────────────┘
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┌──────────────────────────────────────────────────┐
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│ Hardware Infrastructure │
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└──────────────────────────────────────────────────┘
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(Docker based architecture)
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</pre>
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Couple of terms we will encounter frequently are Docker Images and Docker
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Containers. Images are packages or templates of containers all stored in a
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container registry such as [Docker Hub](https://hub.docker.com/). Containers
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are standalone, executable instances of these images which include code,
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runtime, system tools, system libraries and settings - everything required to
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get the software up and running. Coming to Docker, it follows a client-server
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architecture wherein the CLI client communicates with the server component,
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which here is, the Docker Engine using RESTful API to issue commands.
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## The Docker CLI
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```bash
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# after installing Docker from https://docs.docker.com/get-docker/
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# To list available commands, either run `docker` with no parameters or execute
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# `docker help`
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$ docker
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>>> docker [OPTIONS] COMMAND [ARG...]
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docker [ --help | -v | --version ]
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A self-sufficient runtime for containers.
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Options:
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--config string Location of client config files (default "/root/.docker")
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-c, --context string Name of the context to use to connect to the daemon (overrides DOCKER_HOST env var and default context set with "docker context use")
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-D, --debug Enable debug mode
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--help Print usage
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-H, --host value Daemon socket(s) to connect to (default [])
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-l, --log-level string Set the logging level ("debug"|"info"|"warn"|"error"|"fatal") (default "info")
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--tls Use TLS; implied by --tlsverify
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--tlscacert string Trust certs signed only by this CA (default "/root/.docker/ca.pem")
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--tlscert string Path to TLS certificate file (default "/root/.docker/cert.pem")
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--tlskey string Path to TLS key file (default "/root/.docker/key.pem")
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--tlsverify Use TLS and verify the remote
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-v, --version Print version information and quit
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Commands:
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attach Attach to a running container
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# […]
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$ docker run hello-world
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# `docker run <container-name>` is used to run a container, it will pull the
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# images from Docker Hub if they don't already exist in your system. Here the
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# docker client connects to the daemon which in turn pulls the "hello-world"
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# image from the Docker Hub. The daemon then builds a new container from the
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# image which runs the executable that produces the output streamed back to the
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# client that we see on our terminals.
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$ docker run -d ubuntu sleep 60s
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# The -d (or --detach) flag is when we want to run a container in the background
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# and return back to the terminal. Here we detach an ubuntu container from the
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# terminal, the output should be the id and the command exits. If we check
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# running containers, we should still see ours there:
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# CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
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# 133261b4894a ubuntu "sleep 60s" 3 seconds ago Up 2 seconds vigorous_gould
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$ docker run <container-id> -p 3000:8000
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# The -p (or --publish) flag is used to expose port 8000 inside the container to
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# port 3000 outside the container. This is because the app inside the container
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# runs in isolation, hence the port 8000 where the app runs is private to the
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# container.
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$ docker run -i
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# or
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$ docker run -it
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# Docker runs our containers in a non-interactive mode i.e. they do not accept
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# inputs or work dynamically while running. The -i flag keeps input open to the
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# container, and the -t flag creates a pseudo-terminal that the shell can attach
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# to (can be combined as -it)
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$ docker ps -a
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# The `docker ps` command only shows running containers by default. To see all
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# containers, use the -a (or --all) flag
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# Running the above command should output something similar in the terminal:
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# CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
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# 82f84bf6912b hello-world "/hello" 9 minutes ago Exited (0) 9 minutes ago eloquent_sammet
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$ docker stop hello-world
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# or
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$ docker start hello-world
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# The stop command simply stops one or more containers, and the start command
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# starts the container(s) up again! `docker start -a ubuntu` will attach our
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# detached container back to the terminal i.e. runs in the foreground
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$ docker create alpine
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# `docker create` creates a new container for us with the image specified (here,
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# alpine), the container does not auto-start unlike `docker run`. This command
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# is used to set up a container configuration and then `docker start` to shoot
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# it up when required. Note that the status is "Created":
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# CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
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# 4c71c727c73d alpine "/bin/sh" 29 seconds ago Created naughty_ritchie
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$ docker rm 82f84
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# Removes one or more containers using their container ID.
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# P.S.: we can use only the first few characters of the entire ID to identify
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# containers
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$ docker images
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# Displays all images and their information, created here means the latest image
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# tag updated on Docker Hub:
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# REPOSITORY TAG IMAGE ID CREATED SIZE
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# ubuntu latest a8780b506fa4 9 days ago 77.8MB
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# alpine latest 9c6f07244728 3 months ago 5.54MB
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# hello-world latest feb5d9fea6a5 13 months ago 13.3kB
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$ docker rmi
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# Removes one or more images from your system which do not have their instances
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# (or containers as we know them) running. If the image has an attached
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# container, either delete the container first or use the -f (or --force) flag
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# to forcefully delete both the container and image.
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$ docker pull busybox
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# The pull command downloads the specified image on our system from Docker Hub.
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$ docker exec -it 7b272 bash
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# This command is used to run a command in the running container's default
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# directory. Here 7b272 was our ubuntu container and the above command would
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# help us interact with the container by opening a bash session.
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$ docker logs <container-id>
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# Displays the information logged by the specified container
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# root@7b27222e4bb7:/# whoami
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# root
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# root@7b27222e4bb7:/# pwd
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# /
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# root@7b27222e4bb7:/# ls
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# bin boot dev etc home lib lib32 lib64 libx3 srv sys tmp usr var
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# root@7b27222e4bb7:/# exit
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# exit
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# More commands can be found at https://docs.docker.com/engine/reference/commandline/docker/
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```
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## The Dockerfile
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The Dockerfile is a blueprint of a Docker image. We can mention the artifacts
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from our application along with their configurations into this file in the
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specific syntax to let anyone create a Docker image of our application.
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### A few things to keep in mind:
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* It is always strictly named `Dockerfile` without any extensions
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* We have to build our custom image on top of some already available Docker base
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image. (there is an empty image called `scratch` which literally lets you build
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an image from scratch)
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* All capitalised commands are part of the syntax, they are not case-sensitive
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but used like a convention
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* Below is a sample Dockerfile but you can read in depth from the [official docs](https://docs.docker.com/engine/reference/builder/).
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```Dockerfile
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FROM <base-image>
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# define base image
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ENV USERNAME='admin'\
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PWD='****'
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# optionally define environmental variables
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RUN apt-get update
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# run linux commands inside container env, does not affect host env
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# This executes during the time of image creation
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COPY <src> <target>
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# executes on the host, copies files from src (usually on the host) to target
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# on the container
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ENTRYPOINT ["some-script.sh"]
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# executes an entire script as an entrypoint
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CMD [<args>,...]
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# always part of dockerfile, introduces entry point linux command e.g.
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# `CMD node server.js`
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# This executes after image creation only when the container from the image
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# is running.
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```
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### Build your images
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Use the `docker build` command after wrapping your application into a Docker
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image to run ( or build) it.
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```bash
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$ docker build <path-to-dockerfile>
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# used to build an image from the specified Dockerfile
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# instead of path we could also specify a URL
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# -t tag is optional and used to name and tag your images for e.g.
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# `$ docker build -t my-image:0.1 ./home/app`
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# rebuild images everytime you make changes in the dockerfile
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```
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## Push your image to DockerHub
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If you want your application's Docker image to be made publicly available for
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any Docker user, you might wanna push it to the [Docker Hub](https://hub.docker.com/) which is a
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registry of Docker images. Make sure you have an account with a username and
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password on Docker Hub.
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When pushing an image to Docker Hub, we must specify our Docker Hub username
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as part of the source image name. We need to create the target image with the
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tag name of username/image-name much like GitHub repositories.
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```bash
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$ docker login
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# to login to Docker Hub using your username and password
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$ docker tag <src-image>[:<src-tag>] <target-image>[:<target-tag>]
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# this tags a local src-image to a public target-image
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# e.g. `docker tag my-sample-app:1.0.0 akshitadixit/my-sample-app`
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# if tags are not specified, they're defaulted to `latest`
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$ docker push <target-image>[:<target-tag>]
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# uploads our image to Docker Hub
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# e.g. `docker push akshitadixit/my-sample-app`
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# this image will be accessible under your profile's repositories as
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# `https://hub.docker.com/r/username/image-name`
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```
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