Following the improvements in #1972 and #2000, it turns out that there are a couple more areas for improvement:

1. The Dockerfile that we use for building container images can instruct the build system to use cache mounts for Pelican's various web frameworks, in addition to Go's cache directories. When developing locally, this reduces build times by persisting those cache directories between builds. When running inside a GitHub Action, this reduces the size of the image layers that need to be cached, because cache mounts do not get preserved.

2. The GitHub Actions for running tests can be modified (read as: fixed) to cache Next.js's cache directories as its own documentation intends. In Pelican's build process, the .next directory is actually being created under {repository_checkout_directory}/web_ui/frontend.