Base image installation moved to separate Dockerfile

[dolsysmith]

The non-Ruby dependencies have been moved to a separate Docker image, from which the scholarspace-app image (the main Dockerfile) inherits. The base image (scholarspace-base) has been pushed to the GHCR, so when building the app image locally, the hosted base image will be used, which should expedite the CI/CD workflow.

To Test

• On your dev server, remove the existing scholarspace-app image then rebuild the latter locally, verifying that the image builds as expected.
• Verify that the app runs as expected.

Post-merge tasks

• I'll set up a new GitHub Action that will be triggered only upon changes to Dockerfile-base, which will build the image and push the updated image to the GHCR.

[dolsysmith]

Note that this PR now also contains a change to the CI-cache workflow file, in order to allow the GitHub Actions runner to log into GHCR and fetch the scholarspace-base image.

I have verified that with that change, the CI worflow runs as expected.

[alepbloyd]

Just tested, all looks good to me!