Add check for root user and network arg in build

Checking if the user executing the script is root, throws an error.

Add the argument '--network host' to the build command is needed on vpn
connections.

Signed-off-by: Nuno Lopes <nuno.lopes@arm.com>
Change-Id: Id39443d92e27f168f49247c6a678a25b63d85582