Use a specific branch with GIT_URL

[matifali]

It would be nice if we could use a specific branch
e.g.

docker run -it --rm \
    -v /tmp/envbuilder:/workspaces \
    -e GIT_URL=https://github.com/coder/coder/tree/matifali/devcontainer \
    -e INIT_SCRIPT=bash \
    ghcr.io/coder/envbuilder

[kylecarbs]

@matifali you can use #refs/heads/<branch> to specify a custom reference!

[kylecarbs]

We should document this.