ci: only build when image-affecting files change

Add path filters so the workflow runs only when the Dockerfile, nginx config,
site assets or .dockerignore change. CI-config, Renovate-config and docs
changes (such as a Renovate bump of a CI action) no longer trigger a build or
release. workflow_dispatch is added for manual runs after pipeline changes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-and-publish.yml

@@ -1,10 +1,31 @@
 name: Build and publish container
 
+# Only run when files that affect the built image change. Updates to CI
+# config, Renovate config, or docs (e.g. a Renovate bump of the checkout
+# action) do not change the image and so do not trigger a build or release.
+# Use workflow_dispatch to run manually after a pipeline change.
 on:
   push:
     branches: [main]
+    paths:
+      - 'Dockerfile'
+      - 'default.conf'
+      - 'index.html'
+      - 'styles.css'
+      - 'script.js'
+      - 'messages.js'
+      - '.dockerignore'
   pull_request:
     branches: [main]
+    paths:
+      - 'Dockerfile'
+      - 'default.conf'
+      - 'index.html'
+      - 'styles.css'
+      - 'script.js'
+      - 'messages.js'
+      - '.dockerignore'
+  workflow_dispatch:
 
 defaults:
   run: