nginx: Update command to generate host config file (#2411)

Currently, instructions to copy the default configuration from the
nginx container to the host uses three commands. This can be done more
efficiently with one command (assuming the presence of "cat" in the
container which should be a safe).

Update the instructions to use the single command.

nginx/content.md

@@ -42,9 +42,7 @@ You can mount your configuration file, or build a new image with it.
 If you wish to adapt the default configuration, use something like the following to get it from a running nginx container:
 
 ```console
-$ docker run --name tmp-nginx-container -d %%IMAGE%%
-$ docker cp tmp-nginx-container:/etc/nginx/nginx.conf /host/path/nginx.conf
-$ docker rm -f tmp-nginx-container
+$ docker run --rm --entrypoint=cat %%IMAGE%% /etc/nginx/nginx.conf > /host/path/nginx.conf
 ```
 
 And then edit `/host/path/nginx.conf` in your host file system.