There's a dedicated Actions runner available for the forge, running on its own VPS. Right now it has two labels available: ubuntu-latest and bookworm-lxc.

ubuntu-latest

The ubuntu-latest label is a good default label for your actions, providing an experience very similar to what you get on Github by default. This is a normal Docker runner, and should be able to run most commands you'd need (and you can always install additional packages, since it's Ubuntu).

bookworm-lxc

This label is more complicated. This runs a mostly-vanilla Debian Trixie LXC image (not Bookworm, actually) configured to allow for arbitrary Docker usage.

Which label should I use?

You should use ubuntu-latest unless you specifically need to run Docker inside your actions. LXC runners on Forgejo are great, but they're isolated much more strongly than Docker runners, and don't get to benefit from any caching, which means that they'll often be slower than you might be able to get with the more default ubuntu-latest label.