Ignore `.git` rather than remove it

[hturner]

Currently we remove .git as part of localscript.sh L28. This is better for users (who do not need to track changes to the container itself) but a pain for us.

We can do better by updating the VS Code settings, adding

"git.ignoredRepositories": [
    "."
]

so that by default, the git repo will be ignored (note I am using "." not /workspaces/r-dev-env to avoid hard-coding the workspace root. Developers working on the container itself just need to delete this setting and reload VS Code to get VS Code to find the git repo again. (Something to document in the developer documentation).

Will also need to add .vscode/settings.json to .gitignore (changes we want to keep should be added to devcontainer.json).

[hturner]

Closed by #265.