GitLab will return an error if you have an upper case letter at the
start of your username.
```
invalid reference format: repository name must be lowercase
```
The built-in `$CI_REGISTRY_IMAGE` variable does the right thing.
Closes https://gitlab.com/veilid/veilid/-/issues/352
Targets have been parallelized so that initial push of the container cache should build the whole build a little quicker, plus the container should now use the cache for more of the build and so speed up normal builds to just the compilation and test of the code that has changed
The `build_cache` target now builds a `build-cache:latest` container that is stored in the GitLab project Container Registry, and then used (if it exists) by the `test_build` target. The `build_cache` task runs under 3 conditions, 1. the container does not exist, 2. if scheduled, 3. if run manually from the Pipelines page in the GitLab interface.
It is recommended that the build is set up to run on a weekly schedule via the `Pipeline schedules` page in GitLab with the schedule of `0 2 * * 6`.
