Fix local-environment ports (#3136)

* Change Dex port in local-environment from 4190 to 5556

Port 4190 is blocked by standards-compliant browsers (e.g. Firefox), as per https://fetch.spec.whatwg.org/#port-blocking.
Port 5556 is used by Dex in its example config files: 745e1114f3/examples/config-dev.yaml (L50)

* Fix upstream in local-environment/oauth2-proxy.cfg

http://httpbin.localtest.me:8080 is only exposed to the host, not to httpbin Docker network.
Causes Bad Gateway before.

* Do not expose unauthenticated httpbin service in local-environment

This defeats the point of having oauth2-proxy.
It has already been misleading by causing the bug fixed in cafc6af48fc38f6fe4395fb0c7e2638bc84e6091.
It serves as a bad example: users might accidentally expose the service they're trying to protect in the first place.

* Remove unnecessary httpbin.localtest.me alias from local-environment

contrib/local-environment/docker-compose-nginx.yaml

@@ -62,7 +62,7 @@ services:
       - "./dex.yaml:/dex.yaml"
     restart: unless-stopped
     ports:
-      - 4190:4190/tcp
+      - 5556:4181/tcp
     networks:
       dex:
         aliases:
@@ -73,12 +73,9 @@ services:
   httpbin:
     container_name: httpbin
     image: kennethreitz/httpbin
-    ports:
-      - 8080:80/tcp
+    ports: []
     networks:
-      httpbin:
-        aliases:
-          - httpbin.localtest.me
+      httpbin: {}
   etcd:
     container_name: etcd
     image: gcr.io/etcd-development/etcd:v3.6.2