Move off nixery for test image source

[petewall]

It's really convenient, but it's harder to control and tune. Less stable than other image registries.

[petewall]

Work to be done:

1. Change the FROM fields in the Dockerfile
2. Maybe look at the process for building and pushing images
   Currently, it builds during pushes to main, which makes testing PRs a pain (unless using a custom version)
   Is there a smarter way?