Push to new GitHub Container Registry

[carlpett]

This PR adds a push to the "new" Github Container Registry (I kept the push to the "old" Github Package Registry since I guess people may have workflow dependencies on it?).

The new registry has two benefits:

• Allows for unauthenticated pulls (ie no longer need to authenticate your AM nodes with GitHub to get the image)
• Isn't affected by the bug which appears to break containerd

[prymitive]

According to Github stats nobody is using docker repo on Github, so I was actually considering dropping it. But maybe that will change after Docker Hub enforces authenticated pulls.

[prymitive]

Thanks

[carlpett]

According to Github stats nobody is using docker repo on Github

We've been using it 🙂 But we mirror images we use to an internal registry, so we'll only show up as a single pull (and haven't bumped to 0.71 yet).
I'll admit we didn't find the image on Docker Hub - I somehow missed the reference in the readme and assumed it would've been hosted under the prymitive name on Docker Hub as well...