Set up basic CI

[kolyshkin]

What we need here is:

• golangci-lint
• unit tests
• 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.