Use images to provide python/R runtime environments

[bloodearnest]

Currently, we install the R version and packages from r:v1 and python version from python:v2 into the devcontainer image. These act as the foundation to provide LSP support for editing python and R files using the vscode editor.

However, it should be possible to provide these environments from the opensafely images.

e.g.

docker run --it -d --rm --name r-v2 r:v2 bash  # does nothing, but provides mount points
# note: this is for zfs, looks slightly different for overlayfs
mount=$(docker inspect r-v2 -f '{{.GraphDriver.Data.Mountpoint}}')
sudo mount --bind -ro "$mount/usr/bin/R" /usr/bin/R
sudo mount --bind -ro "$mount/usr/lib/R" /usr/lib/R
... # other directories may also need mounting, e.g. specific libs.

This requires some more PoC work to verify, but if it works, it provides more dynamic versions of the run times, and can support multiple images.

This step could be done on postAttach event, rather than baking in to the image, so would track image versions.

It's still only possibly to provide one version of an R or python version at a time, but that choice can be made per project, rather than once for all projects