Split docker-compose into production (pull) and dev (build) files

docker-compose.yml now pulls the pre-built image from
gitea.labsconnect.se/bobban/scheduletaskmanager-schedule-task-manager:latest
instead of building locally; the old build-from-source file moves to
docker-compose.dev.yml for local testing or before a push pipeline
exists. README documents both paths.

docker-compose.yml

@@ -1,6 +1,6 @@
 services:
   schedule-task-manager:
-    build: .
+    image: gitea.labsconnect.se/bobban/scheduletaskmanager-schedule-task-manager:latest
     container_name: schedule-task-manager
     restart: unless-stopped
     ports: