Consider switching to universal devcontainers image

[StevenMaude]

This is the default image for GitHub Codespaces if none is specified:

"image": "mcr.microsoft.com/devcontainers/universal:linux",

It adds no storage cost, unlike other images, and already has Python installed.

It seems to launch quickly for me, which is presumably because it's cached. Maybe that's no advantage if prebuilds are used.

The research template only uses a Python-specific image because that's what seemed sensible, when I first configured it through VSCode.

[StevenMaude]

One downside of this is that the version of the image or of Python can't be pinned. Whereas with the Python image, you can require 3.11.

The Python image has three benefits:

• we don't get out of sync of versions of Python that run the OpenSAFELY CLI without realising (say the image upgrades to Python 3.12)
• there's a more definite image version to upgrade, because going from a 3.11 to 3.12 image will eventually ensure that any Docker image gets upgraded (see Decide how dev container Docker base image gets its version pinned and upgraded #35)
• it's smaller

[lucyb]

I expect we'll want to install python 3.10, like version 2 of the python action image. So, I think anything that doesn't allow us to specify that exact version will not be a good idea.

We'll also probably want to install the relevant python (and maybe R packages, see #9) into the image, rather than install them at start up, so I expect we'll want to have a custom Dockerfile.

I'm going to close this issue for now, but if I've missed anything please comment or reopen it.