A Docker Container specification file (Dockerfile) can be configured to run a containerized application using either ENTRYPOINT or CMD.

ENTRYPOINT is intended to be used when the user wishes for a container to behave as if it were an application.

More information can be found in the Docker Documentation.

An ENTRYPOINT has two forms, a exec form and a shell form.

Here is an example of the exec form:

ENTRYPOINT ["executable", "param1", "param2", ...]

Here is an example of the shell form:

ENTRYPOINT command param1 param2 ...

The Docker Documentation says the following about the shell form:

The shell form of ENTRYPOINT prevents any CMD command line arguments from being used. It also starts your ENTRYPOINT as a subcommand of /bin/sh -c, which does not pass signals. This means that the executable will not be the container’s PID 1, and will not receive Unix signals. In this case, your executable doesn’t receive a SIGTERM from docker stop <container>.

What I don’t understand is what is the intended purpose of this?

• It seems strange to run a container which has signals disabled – the application running inside the container cannot be stopped gracefully
• It also prevents any command line arguments from being used – which seems to contradict the intended purpose of ENTRYPOINT to match the behaviour of the container to that of an executable