Is CMD in parent docker overriden by CMD/ENTRYPOINT in child docker image?

If you define an ENTRYPOINT in a child image, it will null out the value of CMD as identified in this issue. The goal is to avoid a confusing situation where an entrypoint is passed as args a command you no longer want to run.

Other than this specific situation, the value of ENTRYPOINT and CMD are inherited and can be individually overridden by a child image or even a later step of the same Dockerfile. Only one value for each of these will ever exist in an image with the last defined value having precedence.

ENTRYPOINT doesn't override CMD, they just represent to parts of resulting command and exist to make life easier. Whenever container is started, the command for process 1 is determined as ENTRYPOINT + CMD, so usually ENTRYPOINT is just path to the binary and CMD is a list of arguments for that binary. CMD can also be easily overriden from command line.

So, again, it's just a thing to make life easier and make containers behave just like regular binaries - if you have man container, you can set entrypoint to /usr/bin/man and cmd to man. So if you just start container, docker will execute /usr/bin/man man, but if you run something like docker run man docker, the resulting container command will be /usr/bin/man docker - the entrypoint stays the same, cmd changes, and resulting command to start container is just a simple merging of those.

ENTRYPOINT and CMD are both inherited from parent layers (images) unless overriden, so if you inherit from image X and redefine CMD, you will still have the very same ENTRYPOINT and vice versa. However, as @BMitch mentioned below, changing ENTRYPOINT in child image will effectively reset CMD.