Clair v4 allow configurations to be overridden by environmental variables

For app-sre onboarding it would be nice to be able to override the DB specific config variables (in our case that is currently the connstring for all 3 components).

This would allow us to make use of the secrets directly that get injected into our namespaces when cloud dependencies get added / modified (in our case the RDS instances) instead of having to template them into the config.yaml file like so: https://gitlab.cee.redhat.com/service/app-interface/-/merge_requests/26577/diffs#ded97b1b01e1c834675c614ed156876208c6f6f1_0_19.

As discussed with hdonnay, it'd be nice to make use of the PG prefixed environment variables as the driver already supports.

We should also consider which documentation updates we need to make.