Add helper to verify if the configuration file changed

controllers/nginx-third-party/Dockerfile

@@ -12,7 +12,12 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM gcr.io/google_containers/nginx-slim:0.3
+FROM gcr.io/google_containers/nginx-slim:0.4
+
+RUN apt-get update && apt-get install -y \
+  diffutils \
+  --no-install-recommends \
+  && rm -rf /var/lib/apt/lists/*
 
 COPY nginx-third-party-lb /
 COPY nginx.tmpl /