What is a Dockerfile?

See Dockerfile reference in the Docker docs.

An example

Copied from Best practices for writing Dockerfiles

FROM ubuntu:18.04
COPY . /app
RUN make /app
CMD python /app/app.py

Each instruction creates one layer:

  • FROM creates a layer from the ubuntu:18.04 Docker image.
  • COPY adds files from your Docker client’s current directory.
  • RUN builds your application with make.
  • CMD specifies what command to run within the container.

General pattern

Here is an outline of a typical Dockerfile.

FROM ...

RUN ...
RUN ...
RUN ...

WORKDIR ...

COPY ... ...
RUN ...

ENTRYPOINT ...
CMD ...

What do the pieces do?

FROM

The FROM line specifies the base image, usually Linux flavor (e.g. ubuntu) or a specialized container such as for Python or NodeJS.

If the target image does not exist yet, Docker will download it. On later build or run commands, that image will be reused from the cache.

If you want to download an image independently of using it in a FROM line, you can run:

$ docker pull IMAGE

RUN

There are usually some RUN commands when building the image.

  • This is usually a command to update the APT package listing. This is usually required or the next step will fail. 
      RUN apt-get update
  • Then there is a step to install packages. It is useful to add the -q quiet flag for cleaner output and the -y yes flag to install without asking the user to confirm. 
      RUN apt-get update
  • The RUN command can also be used for other commands such as changing permissions, moving files, or downloading and unzipping a file from a URL.

WORKDIR

Set the working directory to a custom value.

This defaults to ~ - that would be /root.

COPY

There is sometimes a COPY commnd to copy a file or folder into the container.

Source

The source could be a file.

COPY foo.txt /root

Or the contents of a directory. Note - the contents of the directory are copied, not the directory itself.

# Similar to this in Bash: cp bar/* /root
COPY bar /root

Or a directory.

COPY bar /root/bar

Or your entire project.

COPY . /root

Destination

This is often copied to / or /app but ~ can work as well. Note that the user will be root so sudo is not needed and the path to ~ will be /root. Also my linter recommends using an absolute rather than ~.

I’d recommend against copying to / since there are directories there relating to the system (similar to a system not insider a container).

CMD / ENTRYPOINT

Finally there is usually CMD or ENTRYPOINT at the end. This will only be run when the container starts - not during the build image step.

  • CMD sets default command and/or parameters, such as starting a server or entering the Python or Bash shell. The default command can be ignored if overwritten when running the container. 
      CMD echo "Hello world"

      CMD ["/bin/bash"]
  • ENTRYPOINT configures a container that will run as an executable. The entry point will always run even if the command is overwritten. 
      ENTRYPOINT echo "Hello world"

      ENTRYPOINT ["/bin/bash"]

See more info in this post.