Add build argument for policy package in relevant packages

[rdimaio]

Sharing this great idea by @maxnoe, already used in the Rucio containers for BDMS - their Dockerfile for reference: https://gitlab.cta-observatory.org/cta-computing/dpps/bdms/bdms-rucio-images/-/blob/main/Dockerfile?ref_type=heads

We can add two build args POLICY_MODULE_NAME and POLICY_PACKAGE_REQUIREMENT. If they're passed at build time, we install the policy package, otherwise we don't install anything.

The main advantage of doing this: communities that only have the need to add a policy package, without customizing the images any further, wouldn't need to create custom images and could simply pass the policy package as build arg.

Downside: it might be potentially unsafe to allow dynamic installation of any package via pip; i.e. POLICY_PACKAGE_REQUIREMENT could be overridden and cause a malicious package to be installed. I think that it would be very unlikely for this to happen in a community (and even then, I don't expect that just installing a malicious Python package could cause significant damage), but it might be worth considering.

POLICY_PACKAGE_REQUIREMENTcould either point to a package name, or to a local directory - Max provided this example:

docker build -v path/to/repo:/policy-src --build-args POLICY_PACKAGE_REQUIREMENT=/policy-src

For remote installs, we could also consider passing an optional argument for pip's --index-url, so that a community could host their policy package on an index other than PyPI.