Force developers not to run build-docker-image as root

Instead recommend developer run build within the docker group.
This script is non-functional as the root user.

Change-Id: Ib70ab55e5e9bc2e7b8639d5d5ffc32a2b8795058

tools/build-docker-image

@@ -1,5 +1,10 @@
 #!/bin/bash
 
+if [[ $EUID -eq 0 ]]; then
+    echo "This script must not be run as root.  Instead add yourself to the docker group." 1>&2
+    exit 1
+fi
+
 TOPDIR=$(git rev-parse --show-toplevel)
 IMGDIR="$(cd "$(dirname "$0")" && pwd)"