lxc-create in semaphore sometimes fails with
ERROR: Unable to fetch GPG key from keyserver
Which often happens behind proxies. As the default key server is a load
balancer, retry a few times.
Also, cache the container image between runs, and only recreate it when
it is older than a week.
Run build/test in LXC for now, as full nested QEMU is too brittle right
now: https://github.com/semaphoreci/semaphore/issues/37
But this at least runs some tests. It ensures that systemd generally
works in containers, as well as provides some backup results if the main
Ubuntu CI is down.
It should make it easier to tweak them without having access to
SemaphoreCI itself (I'm glad I'm an admin there now but it shouldn't
be necessary to be an admin to add a couple of kludges :-)). More
importantly, changes to the settings will go through review and
be tested before they're applied globally potentially breaking
Semaphore as it happened two days ago.
I'll point Semaphore CI to these scripts once the PR is merged.
