Documenting ENTRYPOINT can empty value of CMD

[sudo-bmitch]

Signed-off-by: Brandon Mitchell git@bmitch.net

- What I did

Documented a behavior where ENTRYPOINT will empty out the value of CMD when CMD is defined from a base image. This behavior has confused others, e.g. this post on stackoverflow and I feel it should be documented somewhere better than an old issue. The original issue where this behavior is described is moby/moby #5147.

- How I did it

This is just a documentation change, no magic here.

- How to verify it

You can see the undocumented behavior with the following Dockerfile:

FROM busybox as base
CMD ["hello", "world"]

FROM base as override
CMD ["hello", "world"]
ENTRYPOINT ["echo"]

FROM base as extended
ENTRYPOINT ["echo"]

$ docker build -t test-cmd-ep --target override .
...
$ docker run --rm test-cmd-ep
hello world
$ docker build -f df.cmd-ep -t test-cmd-ep --target extended .
...
$ docker run --rm test-cmd-ep

$ 

- Description for the changelog

No change.

- A picture of a cute animal (not mandatory but encouraged)

[codecov-io]

Codecov Report

Merging #1523 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1523   +/-   ##
=======================================
  Coverage   55.23%   55.23%           
=======================================
  Files         289      289           
  Lines       19374    19374           
=======================================
  Hits        10702    10702           
  Misses       7977     7977           
  Partials      695      695

[vdemeester]

LGTM 🐸

[silvin-lubecki]

LGTM, thank you @sudo-bmitch !