quadlet: Support systemd style dropin files

[alexlarsson]

For a source file like foo.container, look for drop in named foo.container.d/*.conf and merged them into the main file. The dropins are applied in alphabetical order, and files in earlier diretories override later files with same name.

This is similar to how systemd dropins work, see:
https://www.freedesktop.org/software/systemd/man/latest/systemd.unit.html

Also adds some tests for these

This seems useful for things like:
containers/qm#284
It also allows a non-auto-enabled quadlet, i.e. without an install section, to easily add one using a dropin.

Does this PR introduce a user-facing change?

 * Quadlet now support systemd-style dropins

[rhatdan]

Please update the man page.

[alexlarsson]

@rhatdan Added

[vrothberg]

Lovely idea, LGTM

[openshift-ci]

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[vrothberg]

@ygalblum PTAL

[Luap99]

Could you add more tests, i.e. test for [Service] and [Unit] sections as well as the other quadlet specific sections.

[ygalblum]

Great addition and the code LGTM. But, I do have some comments/questions about the doc and tests.

I think I understand what happened in the override tests - the empty Environment= line in 20-second.conf caused any previously set value to be removed. But I wouldn't have understood it from the man page. I don't know, is this the way this is done also in systemd drop-ins?

Anyhow, I think the man page should elaborate some more on how the drop-in should look like.

Also, I think the tests should include different types of keys. The Environment key is a a key=value list that can appear multiple times. But, there are other types of keys. For example, does the code work correctly for the Image key?

[alexlarsson]

I think I understand what happened in the override tests - the empty Environment= line in 20-second.conf caused any previously set value to be removed. But I wouldn't have understood it from the man page. I don't know, is this the way this is done also in systemd drop-ins?

This is just in general how the systemd syntax works for list-kind of keys. It also works inside a single file. I'm not sure we have a specific doc for it in the quadlet manpage, but anyone used to systemd will know about this, and I believe we link to the systemd docs for this in the quadlet manpage.

I'll add some more tests.

[dougsland]

LGTM not sure what's wrong with CI but seems not specific with this patch.

[rhatdan]

/lgtm