Rust Dev Container template

Template for writing Rust projects with devcontainers on VSCode.

It's basically the official Rust devcontainer template with some recommended extensions and extra features in the devcontainer config:

1. Mounting ~/.ssh folder by default (helpful if you don't have a cross-environment ssh agent set up).
2. Importing your total host global gitconfig, including includes.

I considered turning the latter into a devcontainer feature, but those disallow host-machine operations (note missing initializeCommand hook), presumably for the obvious security implications.