feat: read config dir from P2G_CONFIG environment variable

[DinoChiesa]

This introduces a new environment variable , which tells the console/headless image where it can read the configuration file.
This is handy when running the unmodified image in a serverless container engine like Google Cloud Run.

Using this feature with the docker image would look like this:

docker run --network=host -i \
    -v /path/to/secrets/headless-config.json:/app/mounted-config/configuration.local.json \
    -v /path/to/host/output:/app/output \
    -e P2G_CONFIG=/app/mounted-config \
    docker.io/library/p2g-console-modified