Hm, drone doesn't support AWS CodeCommit where my main repo is hosted, so I don't think I can even consider it.
Had a look at self-hosted GitLab, there is immediately a warning at the top of the installation page that there should be at least 4GB of free memory. I could upgrade my droplet, but I don't really want to. > 4GB RAM + 4GB swap supports up to 100 users but it will be very slow"
It's looking like I will probably go with BuildBot...