[CI]: Cache docker images with GH cache action

[mattdeform]

Summary

To avoid pulling a fresh docker image every CI run cache the image. If an image exists in the cache use it, otherwise if there is a newer image available or image is not in the cache: pull.

Basic example

Something like:

steps:
  - name: Checkout code
  - name: Get Docker image last modified date  # used to invalidate cache if image has been updated
  - name: Cache Docker images
  - name: Load cached Docker image
      if: steps.cache-docker.outputs.cache-hit == 'true'

  - name: Pull Docker image if not cached
    if: steps.cache-docker.outputs.cache-hit != 'true'

  - name: Install dependencies
  - name: Set Maya environment variables and run tests

Motivation

Speed up the CI significantly, and lower bandwidth used.

Notes

• Need to figure out action cache size allowance (docs say 10gig), appear to be maxing out at 500mb.
• Retry this once repo is public