Provide a customization point for policy.json

[hmenke]

Sometimes it is not possible to install podman in the default location, which is in fact a very common scenario on HPC systems. This includes not being able to put anything in the /etc/containers directory, including policy.json.

In this case, the supported alternative is to use ~/.config/containers/policy.json. However, that requires every user to repeat that configuration and also makes it tricky or even impossible to apply centralized changes.

The location of other configuration files can be customized via environment variables (e.g. $CONTAINERS_STORAGE_CONF) or in containers.conf (e.g. seccomp_profile).

Please provide a customization point for signature policies as well.