CI fixes

[ublefo]

Please merge #18 first. This took me weeks to figure out and test, now everything is finally working as they should.

Improvements made:

1. Implemented container caching to bring turnaround time from 52 minutes to 16 minutes after the container is built successfully.
2. Set up redis in the workflow and make sure sidekiq is configured to use the correct URL from the environment if it's present.
3. Upgrade all workflows to the latest versions.
4. Install ruby-lsp in the development environment for the corresponding vscode extension, which is currently already installed in the devcontainer but completely useless due to the gem being missing.
5. Implement better error reporting in database_populator.rb to report the error messages when there's something wrong with the test data. Troubleshooting test db initialization issues should be much easier now.
6. Documented the two new redis related env variables.
7. Skip unit tests for documentation updates.
8. Created a dependabot config to check for github actions workflow updates weekly.

Instruction for reviewers: just check the CI workflows below and they should be all passing now.

[maddernd]

Have these changes been tested on Mac and Windows?

[satikaj]

Unit tests work on Mac. I tested the CI GitHub actions workflow on my fork too for these commits and they all succeed now. The implemented cache functionality also makes it run faster the second time around.

[macite]

This looks very good. If we can get that one new environment variable explained alongside the other environment variables in the README I think... then it should be good to merge.

[macite]

Once this is addressed you can make the PR to the main repo.

[maddernd]

This one looks good now please make upstream.

[ublefo]

Pull request opened upstream: doubtfire-lms#434