Add guide for VPN-in-docker (arriven#144)

docs/docker-vpn.md

@@ -0,0 +1,73 @@
+# Setting up VPN for Docker users
+
+In case of using a dedicated VPS that has banned public IP, a container with
+OpenVPN client can be deployed inside the same network as db1000n is in.
+One of the easy ways to set it up is through the docker compose:
+
+`docker-compose.yml`:
+```yaml
+version: "3"
+services:
+
+  ovpn:
+    image: dperson/openvpn-client
+    cap_add:
+      - NET_ADMIN
+    security_opt:
+      - label:disable
+    # read_only: true
+    restart: unless-stopped
+    volumes:
+      - /dev/net:/dev/net:z
+      - /path/to/dir/with/certificate:/vpn
+    # NOTE: user/password can be defined:
+    # 1. In .ovpn configuration file. In this case put it inside
+    #    /path/to/dir/with/certificate (see above)
+    # 2. In the `command` directive (see below).
+    #    If you are using .ovpn file, then comment out three lines below
+    command:
+      - "-v"
+      - "host;user;password"
+
+  db1000n:
+    image: ghcr.io/arriven/db1000n
+    restart: unless-stopped
+    depends_on:
+      - ovpn
+    network_mode: "service:ovpn"
+```
+
+
+## Run:
+
+```sh
+docker compose up -d
+```
+
+
+You can also run multiple db1000n instances with different VPN
+configurations (i.e. multiple countries):
+
+```yaml
+version: "3"
+services:
+  ovpn1:
+    image: dperson/openvpn-client
+    # ...
+    volumes:
+      - /certificate1:/vpn
+  db1000n1:
+    image: ghcr.io/arriven/db1000n
+    # ...
+    network_mode: "service:ovpn1"
+  ovpn2:
+    image: dperson/openvpn-client
+    # ...
+    volumes:
+      - /certificate2:/vpn
+  db1000n1:
+    image: ghcr.io/arriven/db1000n
+    # ...
+    network_mode: "service:ovpn2"
+  # more services...
+```