Added information about running nginx in debug mode

https://github.com/nginxinc/docker-nginx/issues/55

nginx/content.md

@@ -72,7 +72,8 @@ Out-of-the-box, Nginx doesn't support using environment variables inside most co
 
 Here is an example using docker-compose.yml:
 
-```web:
+```
+web:
   image: nginx
   volumes:
    - ./mysite.template:/etc/nginx/conf.d/mysite.template
@@ -88,3 +89,23 @@ The `mysite.template` file may then contain variable references like this :
 
 `listen       ${NGINX_PORT};
 `
+
+## running nginx in debug mode
+ 
+Images since version 1.9.8 come with `nginx-debug` binary that produces
+complete output when using higher verbosity log levels. It can be used 
+with simple CMD substitution:
+
+```console
+docker run --name my-nginx -v /host/path/nginx.conf:/etc/nginx/nginx.conf:ro -d nginx nginx-debug -g 'daemon off;'
+```
+
+Similar configuration in docker-compose.yml may look like this:
+
+```
+web:
+  image: nginx
+  volumes:
+    - ./nginx.conf:/etc/nginx/nginx.conf:ro
+  command: [nginx-debug, '-g', 'daemon off;']
+```