fix: trigger docker-build workflow after GITHUB_TOKEN push from update.yml

GITHUB_TOKEN pushes do not trigger downstream workflows, so docker-build.yml
never fires after update.yml commits a new version. Add a workflow_run trigger
so docker-build runs after Check Update completes.

A change-detection step skips the build when triggered via workflow_run but
buildinfo.json was not modified, avoiding spurious builds on every hourly run.

References #604

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

Author: carlbennett

.github/workflows/docker-build.yml

@@ -6,7 +6,13 @@ on:
       - master
     paths:
       - buildinfo.json
-  # workaround for #526
+  # workflow_run is used because GITHUB_TOKEN pushes from update.yml don't trigger push events
+  workflow_run:
+    workflows: ["Check Update"]
+    types:
+      - completed
+    branches:
+      - master
   workflow_dispatch:
 
 jobs:
@@ -16,12 +22,25 @@ jobs:
 
       - name: Checkout
         uses: actions/checkout@v6
+        with:
+          fetch-depth: 2
+
+      - name: Check if buildinfo.json changed
+        id: check
+        if: github.event_name == 'workflow_run'
+        run: |
+          if git diff --name-only HEAD~1 HEAD | grep -q '^buildinfo.json$'; then
+            echo "changed=true" >> $GITHUB_OUTPUT
+          else
+            echo "changed=false" >> $GITHUB_OUTPUT
+          fi
 
       - name: Set up QEMU
+        if: github.event_name != 'workflow_run' || steps.check.outputs.changed == 'true'
         uses: docker/setup-qemu-action@v3
 
       - name: build and push all images
-        if: ${{ env.DOCKER_USERNAME != '' && env.DOCKER_PASSWORD != '' }}
+        if: (github.event_name != 'workflow_run' || steps.check.outputs.changed == 'true') && env.DOCKER_USERNAME != '' && env.DOCKER_PASSWORD != ''
         env:
           DOCKER_PASSWORD: ${{ secrets.DOCKER_PASSWORD }}
           DOCKER_USERNAME: ${{ secrets.DOCKER_USERNAME }}