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Docker 101

This tutorial is based largely on the excellent Docker Getting Started materials.

Prerequisites

  • Access to a system running Docker OR
  • Access to a Labs Workbench account, where you can run Docker in the cloud

If you haven't already, clone this repo:

git clone https://github.com/nds-org/developer-tutorial.git

What you'll learn

In this tutorial, you'll learn how to:

  • Run an existing Docker image
  • Modify and commit changes to an image
  • Build an image from a Dockerfile
  • Push an image to Dockerhub

Basic Usage

These are just the basics of using Docker.

See the Docker tutorials and "docker --help" for more information.

Finding images on Dockerhub

Look at the Dockerfile -- it is a set of instructions used to build the image:

FROM debian:jessie

RUN apt-get update -y && apt-get install -y cowsay

ENV PATH $PATH:/usr/games

CMD ["cowsay", "Hi"]
  • FROM: This is the base image used by the image. Base images can be any Linux flavor including Debian, Ubuntu, and CentOS.
  • RUN: Installs the ubiquitous "cowsay" utility
  • ENV: Sets the PATH environment variable
  • CMD: The default command for running containers

Starting a container from an existing image

Open a shell/terminal and run:

$ docker run --rm ndslabs/cowsay

The first time you run a container, the image and any dependencies are downloaded. You should see output similar to:

Unable to find image 'ndslabs/cowsay:latest' locally
latest: Pulling from ndslabs/cowsay
73e8d4f6bf84: Pull complete
040bf8e08425: Pull complete
6649a5e013b4: Pull complete
93a35b983d41: Pull complete
7f30ada4c040: Pull complete
Digest: sha256:9501c74dbc9c6f7b67764071476bcacbf7e5a18b0857d96b46920f0431493e0b
Status: Downloaded newer image for ndslabs/cowsay:latest
 ____
< Hi >
 ----
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

The next time you run the command, the images are cached, so you should simply see:

$ docker run --rm ndslabs/cowsay
 ____
< Hi >
 ----
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

Note: The "--rm" flag tells Docker to delete the container when execution completes. If you don't do this, the containers will stick around after exiting. You can see this with the "docker ps -a" command.

You can see the images in your image cache using "docker images":

$ docker images

REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
ndslabs/cowsay      latest              7f30ada4c040        35 minutes ago      170.9 MB

Creating your own Dockerfile

Another way to change an image is to create a new Dockerfile. In this example, we'll combine our efforts above.

On your local system, create a new file called Dockerfile:

FROM debian:jessie

RUN apt-get update -y && apt-get install -y fortune cowsay

ENV PATH $PATH:/usr/games

CMD fortune -a | cowsay

We'll add the fortune package and change the default command

Building the image

Once you have a Dockerfile, you can build a local image:

$ docker build -t fortunecow .

There's too much output to post here, but you should see the 4 steps in the Dockerfile. You can see the resulting image by:

$ docker images

REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
fortunecow          latest              3ac3a1461552        19 seconds ago      172.9 MB
fortune             latest              09e96c21ffa1        5 minutes ago       175.9 MB
ndslabs/cowsay      latest              7f30ada4c040        48 minutes ago      170.9 MB

And you can run a container based on the new image:

$ docker run --rm fortunecow
 ______________________________________
< Today is what happened to yesterday. >
 --------------------------------------
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

Sharing your images

If you have an account on Docker Hub, you can push this image to make it available to NDS Labs and others to reuse.

$ docker tag IMAGE USERNAME/IMAGE[:VERSION]
$ docker login
$ docker push USERNAME/IMAGE[:VERSION]

NOTE: If VERSION is not specified, "latest" is assumed.

For example:

$ docker tag fortunecow <user>/fortunecow
$ docker push <user>/fortunecow

Advanced Usage

Depending on how you plan to use Docker, you may find the following tip useful.

See the Docker tutorials and "docker --help" for more information.

Running a different command

The CMD instruction specifies only the default command. You can run any other command -- as long as it's installed in the container:

For example:

$ docker run --rm ndslabs/cowsay cowsay Hello
 _______
< Hello >
 -------
        \   ^__^
         \  (oo)\_______
            (__)\       )\/\
                ||----w |
                ||     ||

Or even:

$ docker run --rm ndslabs/cowsay ls /usr/games
cowsay
cowthink

Connecting to a container

Sometimes you might want to connect or "shell" into a running container.

$ docker exec -it ndslabs/cowsay sh

This starts an interactive shell. You can now run commands, install packages, edit files, etc. But remember -- since the container was started with "--rm", when you exit your changes will be lost!

Official resources

Additional resources