We are looking to add Java pre-commit hooks to some repos[1] and,
unsurprisingly, the Java-ecosystem linters are written in Java. The
pre-commit image currently has no JRE installed in it but alpine package
one and so adding one is easy enough.

While we're here, bump the Python image version used for pre-commit v4
and provide a default for the `PYTHON_VERSION` build arg to squash a
Docker build warning. Keep pre-commit v3 as is because that image should
only be being used by older, possibly sparsely maintained, CI jobs and
we don't want to switch the Python version out from under them.

[1] https://gitlab.developers.cam.ac.uk/uis/devops/hr/hr-general/-/issues/29