Avoid drift configuration drift between ci-xxx and packages/vfx dockerfiles

[jfpanisset]

The Dockerfiles used to build the ci-xxx images are autogenerated from ci-xxx/image.yaml and are used to build the ci-xxx CI build images for the ASWF + USD projects.

But when building those projects to include as dependencies in higher level ci-xxx images or for the all-inclusive ci-vfxall project, we use the pre-baked Dockerfiles in packages/vfx1 and packages/vfx2

This can result in drift between these build environments, as well as violating the DRY principle.

We should figure out a way to have a single, authoritative definition for the build environment of a package that gets used in both cases.