Feature req: Add options file for gluetun so that you can customize the VPN providers settings

I was really fed up with Private Internet Access always connecting me with Chile or Malaysia or some far away country. I also use jdownloader over this VPN and my.jdownloader.org was totally unusable over these slow far away VPNs.

I edited the docker file like this to archive my goal (solution at the end):

  # Gluetun is our VPN, so you can download torrents safely
  gluetun:
    image: qmcgaw/gluetun:v3
    container_name: gluetun
    cap_add:
      - NET_ADMIN
    devices:
      - /dev/net/tun:/dev/net/tun
    ports:
      - 8888:8888/tcp # HTTP proxy
      - 8388:8388/tcp # Shadowsocks
      - 8388:8388/udp # Shadowsocks
      - 8080:8080/tcp # gluetun
    volumes:
      - ${INSTALL_DIRECTORY}/config/gluetun:/config
    environment:
      - VPN_SERVICE_PROVIDER=${VPN_SERVICE}
      - VPN_TYPE=openvpn
      - OPENVPN_USER=${VPN_USER}
      - OPENVPN_PASSWORD=${VPN_PASSWORD}
      - OPENVPN_CIPHERS=AES-256-GCM
      - SERVER_REGIONS=Andorra,Belgium,Bosnia and Herzegovina,Bulgaria,Croatia,Cyprus,Czech Republic,DK Copenhagen,DK Streaming Optimized,ES Madrid,ES Valencia,Estonia,FI Helsinki,FI Streaming Optimized,France,Georgia,Greece,Hungary,IT Milano,IT Streaming Optimized,Iceland,Ireland,Isle of Man,Israel,Latvia,Liechtenstein,Lithuania,Luxembourg,Monaco,Montenegro,NL Netherlands Streaming Optimized,Netherlands,North Macedonia,Norway,Poland,Portugal,Romania,SE Stockholm,SE Streaming Optimized,Serbia,Slovakia,Slovenia,Switzerland,Turkey,UK London,UK Manchester,UK Southampton,UK Streaming Optimized,Ukraine
    restart: unless-stopped

PIA uses SERVER_REGIONS, other VPNs use different variables. So I think a generic file that is included in the dockerfile would be best. But I really don't know what the best option is. You are the devs ;-)

Keep up the great work! Thanks!