[WIP] Significantly reduce Docker image size

[will-cromar]

This PR brings the size of the GPU image down to 6.24 GB from 17.2 GB

• Use local xla directory instead of cloning from github. This allows building an image from a local working copy
• Separate build image from release image so we don't release build artifacts
• Break build steps up to take better advantage of caching to make debugging less painful. In particular, build TF in its own step because it takes forever
• Remove unnecessary dependencies
• Use smaller python:X.Y-slim base images

TODO:

• [ ] Add torchvision
• [ ] Add shallow clone of tests, example models, etc
• [ ] Add support for patches
• [ ] Update this branch and test again

Needs follow-up:

• Create new GCB config for this image
• Collect wheels and upload to GCS
• Start using in e2e tests