Generate `RequiresMountsFor` for `Type=bind` volumes

[alaviss]

Feature request description

When using Quadlet, it's possible to abstract away a bind mount with a volume, like this:

data.volume:

[Volume]
Type=bind
Device=/var/lib/special-data

program.container:

[Container]
Image=docker.io/alpine:latest
Exec=tail -f /dev/null

Mount=data.volume:/data:z

This would allow shipping quadlet definitions in a system image, then on provisioning add drop-ins to override where the host mount point is (or to use regular managed volumes).

However, the generated volume units do not contain RequiresMountsFor (would be RequiresMountsFor=/var/lib/special-data in this case), which meant depending .container files will not wait for the referenced bind mount location to be available before starting.

Suggest potential solution

For Type=bind .volume, add the value of Device to RequiresMountsFor if it's an absolute path.

Have you considered any alternatives?

This can be manually done by adding RequiresMountsFor to the [Unit] section of .volume files, but it is prone to being out-of-sync with the actual Device definition.

Additional context

No response

[Luap99]

cc @ygalblum

[ygalblum]

@alaviss Thanks for bringing this up. Would you care to open a PR?