Stop build if running under root

Running as root messes up the file permission. Disable it to prevent future support issue.

rbuild

@@ -7,6 +7,7 @@ EXIT_UNSUPPORTED_OPTION=3
 EXIT_SUDO_PERMISSION=4
 EXIT_SHRINK_NO_ROOTDEV=5
 EXIT_DEV_SHM_TOO_SMALL=6
+EXIT_RBUILD_AS_ROOT=7
 
 error() {
     case "$1" in
@@ -30,6 +31,12 @@ error() {
         $EXIT_DEV_SHM_TOO_SMALL)
             echo "Your /dev/shm is too small. Current '$2', require '$3'." >&2
             ;;
+        $EXIT_RBUILD_AS_ROOT)
+            cat << EOF >&2
+You are running $(basename "$0") with root permission, which is not supported.
+If you need root permission to run docker, please add your account to docker group, reboot, and try again.
+EOF
+            ;;
         *)
             echo "Unknown error code $1." >&2
             ;;
@@ -512,6 +519,11 @@ LANGUAGE="C"
 
 SCRIPT_DIR="$(dirname "$(realpath "$0")")"
 
+if (( EUID == 0 ))
+then
+    error $EXIT_RBUILD_AS_ROOT
+fi
+
 if command -v notify-send >/dev/null
 then
     NOTIFY_SEND=notify-send