Make PUBLIC_URL and REACT_APP_SERVER configurable (#266)

* Adjust Dockerfile to include args in build process
* Adjust README.md

README.md

@@ -66,6 +66,9 @@ or by editing it in the [.env](.env) file. See also the
         context: https://github.com/Awesome-Technologies/synapse-admin.git
         # args:
         #   - NODE_OPTIONS="--max_old_space_size=1024"
+        #   # see #266
+        #   - PUBLIC_URL="/synapse-admin"
+        #   - REACT_APP_SERVER="https://matrix.example.com"
       ports:
         - "8080:80"
       restart: unless-stopped