Set default listening port to 80

The v1.10.1 of this image listens on port 80. v1.10.2 makes the port
configurable but listens on port 8080 by default. Since most Cilium tests
using that image are testing port 80, we should keep that as the default
to ease the image update.

Signed-off-by: Paul Chaignon <paul@cilium.io>

Dockerfile

@@ -1,4 +1,4 @@
 FROM k8s.gcr.io/echoserver:1.10
 RUN sed -i 's/listen 8080/listen [::]:PORT ipv6only=off/g' /etc/nginx/nginx.conf
-ENV PORT 8080
+ENV PORT 80
 ENTRYPOINT sed -i "s/PORT/${PORT}/g" /etc/nginx/nginx.conf && /usr/local/bin/run.sh