Add CI tests to ensure valid .containers

[it-a-me]

It looks like you can test individual directories of quadlets with the QUADLET_UNIT_DIRS environment variable. For example

mkdir out && QUADLET_UNIT_DIRS="$(pwd)" /lib/systemd/user-generators/podman-user-generator ./out

generates service files in ./out from all quadlets files in the current directory. It exits with a nonzero exit code if the .quadlet is invalid

[redbeardymcgee]

I haven't had enough time to explore CI, but I like this idea a lot.

[redbeardymcgee]

I've finally found time to start learning this in a new branch. Thanks again for the suggestion.