This will create a python venv and point sasl-xoauth2-tool to use
this venv. This way, when the user executes the command, it will
"automagically" work instead of failing.
So, alpine will download platform-specific cache files, meaning that
when another platform tries to use the cache, it will fail with
packages not being found.
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.
