Dockerfiles: Separate Autoware and Zephyr stages

This allows to build two different images if needed, but without
breaking the current interface.

Parametrize the base image name while retaining the current default.

Add a way to set the user inside of the container to be the same as on
the host while keeping the default of being root.

Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>