Allow to customize app host/port (#181)

Allow to use custom hostname and/or port value for app container from the Web
container. Usefull for some people using containers provider (eg. #142)

web/mattermost-ssl

@@ -34,7 +34,7 @@ server {
         proxy_buffers 256 16k;
         proxy_buffer_size 16k;
         proxy_read_timeout 600s;
-        proxy_pass http://app:80;
+	proxy_pass http://{%APP_HOST%}:{%APP_PORT%};
     }
 
     location / {
@@ -51,7 +51,7 @@ server {
         proxy_buffers 256 16k;
         proxy_buffer_size 16k;
         proxy_read_timeout 600s;
-        proxy_pass http://app:80;
+	proxy_pass http://{%APP_HOST%}:{%APP_PORT%};
     }
 }