GHA Docker build flow for PR's

[bandish-shah]

What does this PR do?

Starting to move Docker builds to GHA. In this PR we:

• Create the initial reusable Docker build workflow (docker-build.yaml)
  • Takes an image config YAML as input
  • Add's supporting script (gen_docker_matrix.yaml) to translate an image config YAML and translate it to a GHA matrix
  • Builds and optionally pushes images to a specified Docker repo
  • Requires repo DockerHub secrets
  • Reusable workflow still needs some work to be completely generic, cleanup will be handled in subsequent PR's as top-level pipelines are developed.
• Create the top level PR Docker workflow (pr-docker.yaml)
  • Runs in PR's whenever repo Docker or Docker build CI files are touched
• Updates image config generation script, adds IMAGE_NAME parameter
• Upgrades flash-attn to 0.2.8
• Testing for this PR:
  • Docker builds pass in GHA
  • Confirm images are pushed successfully to staging repo
  • Confirm image cache's are generated based on IMAGE_NAME
  • Confirm image cache works by re-triggering workflow after initial successful build, confirm speed-up
  • Manually verify pytest test suite works on Torch 1.13 images
• Outside the scope of this PR:
  • We don't confirm that the pytest test suite works with all new images, this will be tested in a follow-up PR that merges the Docker build with the pytest testing workflows
  • Releasing Docker images to prod DockerHub repo's: mosaicml/pytorch, mosaicml/pytorch_vision, mosaicml/composer

What issue(s) does this change relate to?

Fixes CO-1064
Fixes CO-1606
Fixes CO-1762

To Do's Before Submitting

• Ask IT to create a mosaicml/ci-staging repo, migrate off of mosaicml/jenkins-staging
• Disable Jenkins Docker CI flows

Before submitting

• Have you read the contributor guidelines?
• Did you run the tests locally to make sure they pass?
• Did you run pre-commit on your change? (see the pre-commit section of prerequisites)

[dakinggg]

LGTM, proof is in the pudding

[mvpatel2000]

Holding review until EFA and GPU test lands