Request: Make use of environmental variables in docker-compose.yaml

[Katorone]

Using environmental variables will allow users to pull this git without having to reconfigure their settings. An .env file can be used to store these settings.
https://docs.docker.com/compose/environment-variables/

Proposed changes:

environment:
  OVPN_SERVER_NET: ${ENV_OVPN_SERVER_NET}
  OVPN_SERVER_MASK: ${ENV_OVPN_SERVER_MASK}
volumes:
  - ${ENV_BASEDIR}/easyrsa_master:/etc/openvpn/easyrsa
  - ${ENV_BASEDIR}/ccd_master:/etc/openvpn/ccd

environment:
  OVPN_DEBUG: ${ENV_OVPN_DEBUG}
  OVPN_VERBOSE: ${ENV_OVPN_VERBOSE}
  OVPN_NETWORK: ${ENV_OVPN_NETWORK}
  EASYRSA_PATH: ${ENV_EASYRSA_PATH}
  OVPN_SERVER: ${ENV_OVPN_SERVER}
  OVPN_INDEX_PATH: ${ENV_OVPN_INDEX_PATH}
volumes:
  - ${ENV_BASEDIR}/easyrsa_master:/mnt/easyrsa
  - ${ENV_BASEDIR}/ccd_master:/mnt/ccd

The environmental variables can be documented and edited in the .env file. It allows users more flexibility to how they organize and deploy the openvpn-server. For instance, I don't like that the volumes are in the same directory that the git clone is in, so I'd prefer to have a structure like:

basedir
|_ .env
|_ easyrsa_master
|_ ccd_master
|_ ovpn-admin
    |_ docker-compose -p openvpn-master --env-file ../.env up -d --build

Having a flat configuration file that's outside of the git repo directory allows users to more easily pull changes without having to manually edit the docker-compose.yaml. An adjusted start.sh could take the path to the .env file, add any missing variables and alert users of important changes.

I'd be more than happy to do a pull request with these proposed changes, though I'm by no means an expert in either docker, bash or git.