PoC: Use MS devcontainer base image

[Jongmassey]

re #147

Base on MS devcontainer Jammy image to match OpenSAFELY base image.

Install R/Python and their dependencies.

Use docker cp to copy the R/Python library files to the host filesystem which avoids any of the previously seen issues with mounting a running container, however the issues we saw may not apply now we're installing the executables in the host system rather than trying to mount them and their dependencies.

Mounting the libraries appears to be faster still, but there may be some more rough edges that are revealed with further testing. A sample implementation is included but commented out.

[Jongmassey]

Fired up a new codespace based on this branch - got into a working vs code environment in about a minute where I could write ehrql with full syntax highlighting and autocomplete.

The rest of postcreate.sh (which sets up the full Python and R environments) didn't finish until 6 mins later which is quite slow but at least everything else is responsive in the meantime.

I've checked that the right version of Python loads (3.10) and all the expected libraries are available (help("modules")), and similarly with R.

I've added an auto-starting Rstudio instance via opensafely launch to postAttach.sh, but this won't run til after postCreate.sh is done. We could change the ordering of this, or leave it to users to start it themselves, which they could do without having to wait for all of postCreate.sh to finish.