first attempt to source ros properly

Here's two preferential changes to deal with some Docker pains I was having:

1. Only mounts the src folder into the docker. I talked with @amarburg about this last week and I guess I don't understand why you require that the build happen outside the docker. For my purposes, I can't do this, because I'm running 20.04 on bare metal, with no ROS2 installation.
2. Replace Zenoh script with a more comprehensive sourcing/entrypoint script that fails gracefully if the setup script doesn't exist.
3. The dockerignore file is tough to work with. It's quite confusing, when things don't exist inside the container. And again, it seems like it breaks the ability to run on anything other than 24.04 with a local ROS2 build capable of fully building the workspace. Which often doesn't exist and seems to be an unnecessary requirement.

Basically opening this MR as a convo with @amarburg. This could also be expanded to include @lauralindzey's pain points with multiple containers.