fix: derive registry host from gitea.server_url instead of unset variable

REGISTRY_URL var was empty so docker login fell through to Docker Hub.
Now strips protocol from gitea.server_url to get the registry hostname —
no manual variable needed. docker-compose defaults to the known host.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.yml

@@ -1,6 +1,6 @@
 services:
   web:
-    image: ${REGISTRY_URL}/josh/sixflagssupercalendar:latest
+    image: ${REGISTRY_URL:-gitea.thewrightserver.net}/josh/sixflagssupercalendar:latest
     build:
       context: .
       dockerfile: Dockerfile