Dockerfile with entrypoint only from base image

Question

I have a very simple Dockerfile like the below :-

FROM my-base-image
COPY abc.properties /opt/conf/

Now my base-image has a docker entrypoint (at the end of its Dockerfile) but this resulting image as you can see has none. Does this work or do we need to have a docker entrypoint/CMD in any given Dockerfile. Also what would be the order of execution for the COPY instruction in the resulting image. With this i mean since this Dockerfile has no entrypoint it would execute one from the base image but will that be executed after the COPY instruction or will the base image entrypoint be executed first and then execute this COPY instruction when the container starts Just looking for concepts in docker.

Answer 1

Several of the Dockerfile directives (notably ENTRYPOINT and CMD, but also EXPOSE, LABEL, and MAINTAINER) just set metadata in the image; they don't really do anything themselves. Within a single Dockerfile this will work just fine:

FROM ubuntu:18.04
WORKDIR /app
# Just remembers this in the image metadata; doesn't actually run it
CMD ["/app/main.sh"]
# ...we should actually copy the file in too
COPY main.sh /app

When you have one Dockerfile built FROM another image it acts almost entirely like you ran all of the commands in the first Dockerfile, then all of the commands in the second Dockerfile. Since CMD and ENTRYPOINT just set metadata, the second image inherits this metadata.

Building and running an image are two separate steps. In the example you show, the COPY directive happens during the docker build step, and the base image's command doesn't take effect until the later docker run step. (This is also true in Docker Compose; a common question is about why Dockerfile steps can't connect to other containers declared in the Compose YAML file.)

There is one exception, and it's around ENTRYPOINT. If you have a base image that declares an ENTRYPOINT and a CMD both, and you redeclare the ENTRYPOINT in a derived image, it resets the CMD as well (very last paragraph in this section). This usually isn't a practical problem.