Merge pull request #1974 from wglambert/master

Add required kernel version for haproxy's sysctl flag

haproxy/content.md

@@ -39,7 +39,7 @@ $ docker run -it --rm --name haproxy-syntax-check my-haproxy haproxy -c -f /usr/
 $ docker run -d --name my-running-haproxy --sysctl net.ipv4.ip_unprivileged_port_start=0 my-haproxy
 ```
 
-You may need to publish the ports your HAProxy is listening on to the host by specifying the -p option, for example -p 8080:80 to publish port 8080 from the container host to port 80 in the container. Make sure the port you're using is free.
+You will need a kernel at [version 4.11 or newer](https://github.com/moby/moby/issues/8460#issuecomment-312459310) to use `--sysctl net.ipv4.ip_unprivileged_port_start=0` , you may need to publish the ports your HAProxy is listening on to the host by specifying the -p option, for example -p 8080:80 to publish port 8080 from the container host to port 80 in the container. Make sure the port you're using is free.
 
 **Note:** the 2.4+ versions of the container will run as `USER haproxy` by default (hence the `--sysctl net.ipv4.ip_unprivileged_port_start=0` above), but older versions still default to `root` for compatibility reasons; use `--user haproxy` (or any other UID) if you want to run as non-root in older versions.