[docker] Implement Docker image automated updates

[driazati]

Following on to discussion in #11673 and #11756 (and the original RFC), the tasks remaining to simplify Docker image updates are:

• Auto-push Docker images to tlcpackstaging from post-merge builds on main if there were Docker changes
  - [ci][docker] Remove Docker image upload prefix #11769
• On a nightly GitHub Actions job, push a PR against a nightly-docker-update branch in apache/tvm that uses the latest images from tlcpackstaging (or updates an existing PR if not merged). A committer can merge the PR when CI passes to start using the new images.
  - [ci][docker] Send a PR to bump the Docker images nightly #11813
• CI will try to find the new images in tlcpack and fail, falling back to tlcpackstaging.
  - [ci][docker] Fall back to tlcpackstaging if images don't exist #11775
• Once a fallback build is successful, the images will be re-tagged over to tlcpack using the API mechanism (i.e. no docker pull)
  - [ci][docker] Tag tlcpackstaging images to tlcpack #11832

and corollary

1. For docker/ updates, run the build twice. Once with the old images, and once with the newly built images (so that changes can be landed and tested with one PR)