Split -dx and base Containerfiles and Workflows

[m2Giles]

Currently our -dx images build the base image in the process of building them. With the reusable build workflow we can possible have -dx built directly from the just built base image.

[p5]

I'm in two minds about this...

It would increase the feedback loop in PRs and make us wait around double the time before merging the PRs, however it would reduce the overall time spent on builders since dx would not need to build everything in the base again.

[castrojo]

I bet dx continues to rev fast so it might end up freeing up builders if they were split? I'm curious now lol.

[m2Giles]

#1136

[m2Giles]

There is a blocker for this. -dx builds are now dependent on the underlying image to build and be available. On pull requests we do not publish those images meaning that we will be unable to ensure that nothing in the underlying base has broken -dx.

[m2Giles]

Unless we allow PRs to be published, this is blocked.

[p5]

An alternative could be to split the Containerfiles, but build both in a single workflow/job.

This reduces the total build runner minutes, but adds some separation.

E.g.

1. step to build base
2. optional push to registry
3. step to build DX from local base
4. optional push to registry