simplify reverse proxy (#98)

make use of mainline nginx image, switch ssl/non-ssl config file based on presence of cert/key, hardcode app port since it is hardcoded in the app container as well

web/mattermost-ssl

@@ -32,7 +32,7 @@ server {
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
         proxy_set_header X-Forwarded-Proto $proxy_x_forwarded_proto;
         proxy_set_header X-Frame-Options SAMEORIGIN;
-        proxy_pass http://app:APP_PORT;
+        proxy_pass http://app:80;
     }
 }