REGISTRY_AUTH_FILE setting is ignored

Podman and Docker CLIs both check for a `REGISTRY_AUTH_FILE` environment variable at run time and, if set, use that path to read/write registry credentials. `podman-py` ignores it and only checks the default podman auth file location.

Is there a way to make `podman-py` follow the same credential resolution logic as the CLI? Looking at the docs, it seems like `credstore_env` might be relevant, but details are a little lacking.

The issue names no files or tests; start by reviewing the documentation for `credstore_env` and tracing the authentication entry point in podman-py. Compare its credential resolution with the CLI behavior described here, and consider the work complete when `REGISTRY_AUTH_FILE` controls credential reads and writes as expected.

Written by the indexing model from the issue text.