🏗️ chore: Set Up Docker Buildx for GitNexus Image GHA Cache Export (#12619)

The first GHCR build on main failed with:
  ERROR: Cache export is not supported for the docker driver.

The default docker driver on ubuntu-latest runners can't export
cache to type=gha. docker/setup-buildx-action@v3 without a driver
argument defaults to docker-container, which supports both
cache-from and cache-to. Gated on the same condition as the build
step so it only runs when an image rebuild is actually needed.

.github/workflows/gitnexus-deploy-do.yml

@@ -111,6 +111,10 @@ jobs:
         id: tag
         run: echo "value=v${{ env.GITNEXUS_VERSION }}" >> "$GITHUB_OUTPUT"
 
+      - name: Set up Docker Buildx
+        if: steps.changes.outputs.changed == 'true' || github.event_name == 'workflow_dispatch'
+        uses: docker/setup-buildx-action@v3
+
       - name: Log in to GHCR
         if: steps.changes.outputs.changed == 'true' || github.event_name == 'workflow_dispatch'
         uses: docker/login-action@v3