This refactor simplifies the `Dockerfile` by considerable amount:
new features of mobykit buildkit allow us to cache `apt`/`apk`
folder (so no need to delete `/var/lib/cache`) as well as extract
the build / install logic into separate files, which removes the
need for `&& \` and makes the code *much more* readable.
`build.sh` script has been updated as well to allow caching local
builds when testing, making it much more easier to test.
