Put cfg.entrypoint toEntrypoint instead of Cmd

[Atry]

Currently when the image built by devenv is run with custom arguments, Cmd will be replaced. As a result, the script created by mkEntrypoint would not get executed. For example, the following command will fail because bash is not in the path:

docker run -it my-image bash

even when bash is added to packages:

packages = [pkg.bash];

This PR set Entrypoint to cfg.entrypoint so that is would not be overwritten by docker run arguments.

Entrypoint = cfg.entrypoint;

Now docker run works:

docker run -it my-image bash