Set up basic CI

[kolyshkin]

What we need here is:

• golangci-lint (done in Add CI #3, followups in Switch to golangci-lint v2, fix or suppress new warnings #12)
• unit tests (done in Add CI #3, CI: add cgroup v1 (AlmaLinux 8) #6)
• integration tests with runc@HEAD

Alas, cgroup v1 testing won't be possible since ubuntu-20.04 is being decomissioned soon, but I hope we can set up Cirrus CI (or test cgroups@HEAD in runc repo)

[kolyshkin]

Unit tests are now done (see #3, #6).

Not sure what to do with integration tests -- perhaps we can do this in runc instead. Or we can do it here.

[kolyshkin]

I think we should close this one, and open a new one about testing runc@HEAD against cgroups@HEAD (most probably in runc repo).