I've standardized on getting github actions to create/pull a docker image and run build/test inside that. So if something goes wrong I have a decent live debug environment that's very similar to what github actions is running. For what it's worth.
It has the massive benefit of solving the lock-in problem. Your workflow is generally very short so it is easy to move to an alternative CI if (for example) Github were to jack up their prices for self hosted runners...
That said, when using it in this way I personally love Github actions
I run my own jenkins for personal stuff on top of nixos, all jobs run inside devenv shell, devenv handles whatever background services required (i.e. database), /nix/store is shared between workers + attic cache in local network.
Oh, and there is also nixosModule that is tested in the VM that also smoke tests the service.
First build might take some time, but all future jobs run fast. The same can be done on GHA, but on github-hosted runners you can't get shared /nix/store.
Remember the old meme image about Vim and Emacs learning curves? Nix is both of those combined.
It's like custom made for me on the idea level, declarative everything? Sign me up!
But holy crap I have wasted so much time and messed up a few laptops completely trying to make sense of it :D
Nix specifies dependencies declaritively, and more precisely, than Docker (does by default), so the resulting environment is reproducibly the same. It caches really well and doubles as a package manager.
Despite the initial learning curve, I now personally prefer Nix's declarative style to a Dockerfile
This may also leverage docker (compose) to build/run different services depending on the stage of action. Sometimes also creating "builder" containers that will have a mount point for src and output to build and output the project in different OSes, etc. Docker + QEMU allows for some nice cross-compile options.
The less I rely on Github Actions environment the happier I am... the main points of use are checkout, deno runtime, release please and uploading assets in a release.
It sucks that the process is less connected and slow, but ensuring as much as reasonable can run locally goes a very long way.
The action does nothing other than trigger the hook.
Then my server catches the hook and can do whatever I want.
I do have plans to create a couple libraries in the near future so will have to work through the pain(s)... also wanting to publish to jsr for it.