Run the merge gate only for changes its tests read

.github/workflows/test.yml

@@ -16,12 +16,28 @@ jobs:
       - uses: actions/checkout@v6
         with:
           persist-credentials: false
+          fetch-depth: 0
+      - name: Relevant changes
+        id: changes
+        run: |
+          if [ "$GITHUB_EVENT_NAME" = "pull_request" ]; then
+            git fetch --no-tags origin "$GITHUB_BASE_REF"
+            base="origin/$GITHUB_BASE_REF"
+          else
+            base="HEAD^"
+          fi
+          if git diff --quiet "$base"...HEAD -- '*.go' go.mod go.sum Dockerfile .dockerignore .github data README.md; then
+            echo "code=false" >> "$GITHUB_OUTPUT"
+          else
+            echo "code=true" >> "$GITHUB_OUTPUT"
+          fi
       # `docker build` streams the context to the daemon. A compose bind-mount of
       # `.` mounts an empty host dir instead, because the job is itself a container.
       - name: Latest supported Go
+        if: ${{ steps.changes.outputs.code == 'true' }}
         run: docker build .
       # Runs even when the step above failed, so a red build says whether the
       # failure is version-specific.
       - name: Lowest supported Go
-        if: ${{ !cancelled() }}
+        if: ${{ !cancelled() && steps.changes.outputs.code == 'true' }}
         run: docker build --build-arg GO_VERSION=1.22.12 .