Enhancements to the standalone dev container

[pvetere]

I've decoupled the standalone dev container from the VS Code one. This
allows us to better customize the standalone development experience.

As part of this, I've added a parameter to the script that allows the
user to "target" the container to a particular source directory.
Before, it was only possible to edit the contents of the current repo.

I've also renamed docker-shell.sh to dev-shell to be slightly more
descriptive and concise.

[esoterra]

The idea of keeping them linked as much as possible is a good goal in abstract, but they are different use cases with different dynamics so I think this is a good idea. After they've been separated, we should consider using more of the VS Code magic in the dev container to make things cleaner. They've done a lot to make some things "just work" that I'm hoping will make some weird permissions problems I've seen go away.

[carlsverre]

Building an entire image for each unique UID/GID seems less ideal than just mapping the UID/GID at runtime. Eventually we should push the docker image (and also figure out how to slim it down below 1GB) to help people quickstart.

The way that runtime uid/gid works with docker is like so:

• create a user/group in the docker image with UID 1000 and GID 1000
• use the --user flag with docker run: docker run --user $UID:$GID
• within the container, it will transparently map UID 1000 to $UID outside the container (same with GID)

[pvetere]

@carlsverre Totally agree about not having to bake in the UID/GID. I just couldn't figure out the magic sauce to get it to happen. I'll try the method you suggested.