Use ghcr.io for grammar compiler docker container

[lildude]

Description

Docker has release a very confusing message about the sunsetting of Free Team organizations. We use such a team organisation for storing the grammar compiler docker image and our choice to use this predates the introduction of the GitHub Packages registry.

It's not 100% sure how we'd be affected by this change, but suffice to say it's the motivator we need to stop using it. We don't really need to be using dockerhub, so this PR switches to using using ghcr.io.

This change will be hidden to most users.

[ Checklist doesn't apply ]

[Alhadis]

What's the reason for using Docker anyway? Wouldn't it have been easier to implement the grammar compiler in Ruby?

(I might be asking a dumb question… it's been a while since I've gone near that part of Linguist's codebase…)

[lildude]

When it was written, I believe the intention was to put prettylights into a standalone service and use protobuf for cross-service communication of the grammars so the compiler has the ability to produce these files too. I think this was experimented with and then discarded for some reason in favour of just using the tarball of JSON files directly in treelights.

I wasn't involved in this decision and I have no idea what the highlighter team's intentions are beyond "eventually move everything to treesitter" so I'm just going to keep the lights on for now.