multiuser g-w on linux should not require a GUI

[aerickson]

I tried to make a headless linux g-w multiuser image, but it's failing due to no GDM config.

A GUI should not be required. It adds significant overhead and complexity to the worker image.

Could not read file /etc/gdm3/custom.conf to update auto login user: open /etc/gdm3/custom.conf: no such file or directory

example worker pool with this error (may be fixed at some point as I'll need to rebuild with a GUI to unblock): https://firefox-ci-tc.services.mozilla.com/worker-manager/gecko-1%2Fb-linux-2204-kvm-gcp/errors

[petemoore]

Originally raised as https://bugzilla.mozilla.org/show_bug.cgi?id=1609102 but better to have this tracked as github issue. Thanks for raising.

[aerickson]

This is now causing a significant amount of work that wouldn't be necessary if it didn't require a GUI.

See https://bugzilla.mozilla.org/show_bug.cgi?id=1900768.

[petemoore]

Spotted we have a duplicate here, originally raised as #4595