Set admin credentials with environment variables

[dmenneck]

This project lacked the option of specifying the username and password of the admin user via environment variables. With this PR it's possible to set the admin credentials like so:

podman run -d -p 8081:8080 -e GEOSERVER_ADMIN_USER=admin -e GEOSERVER_ADMIN_PASSWORD=geoserver <image>

The changes are heavily influenced by preliminary work of meggsimum and kartoza.