Ensure /etc/resolv.conf symlink exists by default as it has regressed with overlayfs switch from previous behavior

[invidian]

Description

I'm reporting it as a bug, as it's a regression from earlier Flatcar versions where this issue did not occur.

With image-builder project, as part of building Flatcar CAPI images, we run /usr/share/oem/bin/waagent -force -deprovision+user before shutting down building machine to ensure resulting OS image can be used for booting multiple instances.

Waagent deprovisioning removes /etc/resolv.conf file which with Flatcar version stable 3602.2.2 results in a broken DNS resolution on newly booted machines. This is because /etc/resolv.conf symlink has been removed and there is nothing to re-create it. This hasn't been an issue in the past, as Flatcar was not using overlayfs for /etc/ directory.

Impact

This has broken CAPZ e2e tests, meaning CAPZ deployments for Flatcar are currently broken.

Environment and steps to reproduce

1. Set-up: [ describe the environment Flatcar/Lokomotive/Nebraska etc was running in when encountering the bug; Platform etc. ]
2. Task: [ describe the task performing when encountering the bug ]
3. Action(s): [ sequence of actions that triggered the bug, see example below ]
   a. [ requested the start of a new pod or container ]
   b. [ container image downloaded ]
4. Error: [describe the error that was triggered]

Expected behavior

/etc/resolv.conf file should be recreated by default by Flatcar, for example with a tmpfiles.d rule.

CC @pothos @jepio @tormath1

[pothos]

It can also happen with other files. In Flatcar we have to either add the filtered-out /etc tmpfile rules back (downside is that it creates state in the upperdir) or clean up the overlayfs whiteouts for files that should exist (according to a tmpfile rule) so that they get propagated from the lowerdir.

PR: flatcar/scripts#1442

[pothos]

Merged and picked for Stable, Beta, Alpha