Custom Docker image doesn't inherit CMD

Question

I have an image built using the following Dockerfile:

FROM php:7-fpm
COPY docker-entrypoint.sh /usr/local/bin/docker-entrypoint
RUN chmod +x /usr/local/bin/docker-entrypoint
ENTRYPOINT ["docker-entrypoint"]

It uses a custom entrypoint which is the following bash script:

#!/bin/bash
set -e
echo "STARTING PHP FPM, CMD $@"
docker-php-entrypoint "$@"

The command of the parent image defined in it's Dockerfile is CMD ["php-fpm"]

Now I expected this command to be inherited in my image. However, when I run the image, the command is empty. Here is the output of the docker run command:

STARTING PHP FPM, CMD

However, if I copy CMD ["php-fpm"] to the end of my custom Dockerfile, the output is

STARTING PHP FPM, CMD php-fpm

[01-Nov-2017 20:04:02] NOTICE: fpm is running, pid 7

[01-Nov-2017 20:04:02] NOTICE: ready to handle connections

Why isn't the parent's CMD directive inherited?

Answer 1

This behavior is trying to be more intuitive, but I agree that it's a bit confusing. You see the original issue here. That problem was that most people defining the ENTRYPOINT in a child image no longer wanted the CMD from the parent image.

With the current behavior, if you define an ENTRYPOINT in a child image, the CMD from the parent image will be null'd out, so you'll need to redefine it if you need to have it set.