ci: Switch to mutter's docker image
Mutter's CI now also builds gnome-shell to ensure that a MR doesn't break the shell. Its docker image has therefore been updated to contain all our deps as well, so we don't need our own image anymore.
Mutter's CI now also builds gnome-shell to ensure that a MR doesn't break the shell. Its docker image has therefore been updated to contain all our deps as well, so we don't need our own image anymore.