For Debian, there isn't a lot of value of testing both testing and unstable - they're typically only about 2 weeks apart (except during release freezes, at which point testing doesn't move while unstable does). I think we could get away with only testing one (I imagine you'd pick unstable).
For Fedora, rawhide is going to break a lot, just because it's rawhide. We'd probably have better luck with "fedora:latest".
(I can't speak to Ubuntu, but I imagine similar things apply as in Fedora because they have a similar release cadence.)
For Debian, there isn't a lot of value of testing both testing and unstable - they're typically only about 2 weeks apart (except during release freezes, at which point testing doesn't move while unstable does). I think we could get away with only testing one (I imagine you'd pick unstable).
For Fedora, rawhide is going to break a lot, just because it's rawhide. We'd probably have better luck with "fedora:latest".
(I can't speak to Ubuntu, but I imagine similar things apply as in Fedora because they have a similar release cadence.)