ci: remove pull_request trigger from Docker build workflow

The PR trigger caused Docker builds on every push to PR branches,
wasting CI resources. Only build on merge to main/next-release.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/docker-build.yml

@@ -1,9 +1,9 @@
 # Gitea Actions: Docker Build, Push, and Notify
 #
 # CI/CD pipeline for Major Domo Discord Bot:
-# - Builds Docker images on every push/PR
+# - Builds Docker images on merge to main/next-release
 # - Auto-generates CalVer version (YYYY.MM.BUILD) on main branch merges
-# - Supports multi-channel releases: stable (main), rc (next-release), dev (PRs)
+# - Supports multi-channel releases: stable (main), rc (next-release)
 # - Pushes to Docker Hub and creates git tag on main
 # - Sends Discord notifications on success/failure
 
@@ -14,9 +14,6 @@ on:
     branches:
       - main
       - next-release
-  pull_request:
-    branches:
-      - main
 
 jobs:
   build: