Update nginx version and remove dumb-init

controllers/nginx/rootfs/Dockerfile

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-FROM gcr.io/google_containers/nginx-slim:0.14
+FROM gcr.io/google_containers/nginx-slim:0.15
 
 RUN DEBIAN_FRONTEND=noninteractive apt-get update && apt-get install -y \
   diffutils \
@@ -21,10 +21,4 @@ RUN DEBIAN_FRONTEND=noninteractive apt-get update && apt-get install -y \
 
 COPY . /
 
-# https://blog.phusion.nl/2015/01/20/docker-and-the-pid-1-zombie-reaping-problem
-RUN curl -sSL -o /sbin/dumb-init https://github.com/Yelp/dumb-init/releases/download/v1.2.0/dumb-init_1.2.0_amd64 && \
-  chmod +x /sbin/dumb-init
-
-ENTRYPOINT ["/sbin/dumb-init", "--"]
-
 CMD ["/nginx-ingress-controller"]