build: Cache images in unit tests

[bmtcril]

Description

Another attempt to stop Dockerhub from rate limiting us in CI. If this works here I'll try to add this caching to other relevant workflows.

Supporting information

openedx/axim-engineering#1350 (comment)

Current attempt is using this action to attempt the cache: https://github.com/marketplace/actions/docker-cache

[bmtcril]

Closing and reopening to make sure caching actually works across runs. I'll need to update the cache key after this as it's currently incorrect, but first need to make sure it actually does what it's supposed to.

[bmtcril]

This seems to solve the issue, at least in the unit tests. Previous:

   Starting single-node instance, no replica set
    - port [27017]
    - version [7.0]
    - database []
    - credentials [:]
    - container-name [mongodb]
  
  Unable to find image 'mongo:7.0' locally
  7.0: Pulling from library/mongo
  6414378b6477: Pulling fs layer
  ...

Now:

Cache restored successfully
docker load --input ~/.docker-images.tar
Loaded image: mongo:7.0
...


Starting single-node instance, no replica set
    - port [27017]
    - version [7.0]
    - database []
    - credentials [:]
    - container-name [mongodb]
  
  fd7ffabd6aeb7ef3cc4e361c39e5896c24dbe7578847fa50a1b0b1972cad2b48

[kdmccormick]

Let's try it!

[timmc-edx]

Nice. And even if we do start pulling other Docker images and forget to update the cache key, I don't think that will cause any failures (directly) or correctness issues -- just more risk of rate-limiting again.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.