Because the build system requires that its command are not executed by root,
the user openwrt was created. The buildroot can be found in
/home/openwrt/openwrt.
To run a shell in the buildroot, execute the following command:
docker run -t -i noonien/openwrt-buildroot sudo -iu openwrt bash
More information on how to use this buildroot can be found on the
OpenWRT wiki.