Docker image building tests

Try out different strategies on Github Actions to accelerate Docker image building with Tutor.

Related conversation: https://discuss.openedx.org/t/is-building-docker-images-taking-too-long-for-you/9451
Related GitHub issue: openedx/wg-devops#6

Results

Build "openedx" Docker image from scratch:

Baseline: 35.8 min (2150s) (source). This is the build time that everyone experiences when they build the openedx Docker image for the first time locally.
With registry cache: 9.1 min(546s) (source).
With a registry cache (edx-platform fork): 13.9 min (835s) (source).

Re-build the "openedx" Docker image with an edx-platform fork:

Baseline: 14.86 min (892s) (source). This is the build time that everyone experiences when they need to re-build with an edx-platform fork.
Build-time mount cache: 7.5 min (450s) (source).
Build-time mount cache + registry cache: 10.9 min (650 s) (source).