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Images

The following exercises cover core commands related to working with container images.

NOTE: Pushing a container image to Docker Hub is NOT covered in these exercises. The tutorial should be able to be completed without setting up any sort of external account. For specifics related to pushing an image, see the main Docker documentation on the push command.

Use the below command references to complete the exercises.

Container Image Commands

Command Description
docker images Displays locally cached container images.
docker pull <image name> Pulls an image from a registry.
docker push <image name> Pushes an image to a registry.
docker tag <image name> <new image name> Tags an image with a new image tag.
docker rmi <image tag/image ID> Untags and/or removes an image.
docker build -t <image name> <dockerfile path> Builds an image.

Docker Hub

Command Description
docker login Authenticates to a remote registry.

Dockerfile Reference

For a more thorough list, see the Dockerfile Reference in the main Docker docs

Instruction Description
FROM Specifies which image should be used as its base image.
RUN Executes command(s) within the container.
COPY Copies files and directories from a relative location on the host to the destination within the container image.
ENV Sets environment variables within the container.
EXPOSE Informs Docker that the specified ports should be exposed outside the container.
CMD Provides default command to be run by the container.
ENTRYPOINT Provides default executable for the container.

Less Commonly used Instructions

Instruction Description
ADD Similar to COPY that can fetch remote resources.
ARG Pass arguments to used during the image build.
LABEL Add additional metadata to the image.
ONBUILD Instruction that will be executed when image is used as a base for another image.
SHELL Override default shell.
USER Change User and Group that the container executes as.
VOLUME Flags a directory within the image that should be persisted when a container is started.
WORKDIR Define the working directory.

Exercise 1

Use the pull command to pull an image.

  • Pull the latest version of the alpine image.
  • tag the image as myalpine:latest
  • View all the locally stored images.

[Solution]

Exercise 2

Learn how to build a container image.

  • Update the Dockerfile in this directory with the following changes:
    • Base it off the myalpine image tagged in the previous exercise.
    • Add the environment variable: ENVIRONMENT=dev
    • It should Execute the following additional commands:
      • apk update
      • apk add nginx
    • Copy the file default.conf to /etc/nginx/conf.d/default.conf
    • Expose port 80.
    • Use the following as the default command: nginx -g daemon off; NOTE: daemon off; should be passed as a single parameter
  • Build the image and tag it as mynginx.
  • View all the locally stored images.
  • Start a daemonized container based off the mynginx image you just built. Map port 80 from the container to the host with -p 80:80. Verify you can see the "Welcome to nginx!" page at http://localhost
  • Stop the container.

Container images allow you to bundle up and distribute your application in an easy to consume form. Using a Dockerfile you can build these applications in a compostable and reusable form that can be built off of and extended for a variety of purposes.

[Solution]

Exercise 3

Optimize the container image.

  • Using the Dockerfile from the previous example. Update it to only use a single ENV and RUN command.
  • Rebuild the image calling it the same thing: mynginx
  • View all the locally stored images and note the difference.

In general it is best to build an image with fewer layers that are logically grouped together. These tend to be smaller in size and easier to understand. How many layers and when to break them up is subjective, but as you build and use more, you will find what works best for you and your workflow.

[Solution]

Exercise 4

Remove a container image.

  • Untag (delete) the image with no tag (<none>).

[Solution]