we reuse the same minecraft job/service for all gamemodes and the lobby.
the following variables are supported:
- mode(<string>, required): name of the gamemode to run. this is used to advertise.
- image(<string>, required): image to use, something based on papermc normally.
- plugins(<list(string)>, optional): space-separated list of URLs to download to the plugins folder.

We use images to make gamemode specific changes cause else we'd need a lot of
variables and it'd just become a mess. we still need the plugins variable cause
we don't want to rebuild our images every time we release a new update.
our plugins also are usually unversioned, so we'd break reproducability if we'd
fetch them in the images directly.