add default values for env vars

Dockerfile

@@ -79,6 +79,8 @@ ENV OPENVPN_USERNAME=**None** \
     TRANSMISSION_RPC_AUTHENTICATION_REQUIRED=false \
     TRANSMISSION_RPC_BIND_ADDRESS=0.0.0.0 \
     TRANSMISSION_RPC_ENABLED=true \
+    TRANSMISSION_RPC_HOST_WHITELIST= \
+    TRANSMISSION_RPC_HOST_WHITELIST_ENABLED=true \
     TRANSMISSION_RPC_PASSWORD=password \
     TRANSMISSION_RPC_PORT=9091 \
     TRANSMISSION_RPC_URL=/transmission/ \