Run a command line when starting a docker container

Docker executes RUN commands when you build the image.

Docker executes the ENTRYPOINT command when you start the container. CMD goes as arguments to ENTRYPOINT. Both of these can be overridden when you create a container from an image. Their purpose in a Dockerfile is to provide defaults for the future when you or someone else will be creating containers from this image.

Consider the example:

FROM debian:buster

RUN apt update && apt install procps

ENTRYPOINT ["/usr/bin/ps"]
CMD ["aux"]

The RUN command adds the ps executable to the image, ENTRYPOINT and CMD are not executed but they will be when you run the container:

# create a container named 'ps' using default CMD and ENTRYPOINT
docker run --name ps my_image
# equivalent to /usr/bin/ps aux

# start the existing container 'ps'
docker start ps
# equivalent to /usr/bin/ps aux

# override CMD
docker run my_image au
# equivalent to /usr/bin/ps au

# override both CMD and ENTRYPOINT
docker run --entrypoint=/bin/bash my_image -c 'echo "Hello, world!"'
# will print Hello, world! instead of using ps aux

# no ENTRYPOINT, only CMD
docker run --entrypoint="" my_image /bin/bash -c 'echo "Hello, world!"'
# the output is the same as above

Each time you use docker run you create a container. The used ENTRYPOINT and CMD are saved as container properties and executed each time you start the container.