Adjusted nice permission check to be compatible with debian.

rootfs/etc/cont-init.d/00-app-niceness.sh

@@ -10,9 +10,11 @@ set -u # Treat unset variables as an error.
 APP_NICE_CMD=' '
 
 if [ "${APP_NICENESS:-UNSET}" != "UNSET" ]; then
-    APP_NICE_CMD="nice -n $APP_NICENESS"
+    APP_NICE_CMD="$(which nice) -n $APP_NICENESS"
 
-    if ! $APP_NICE_CMD echo &> /dev/null; then
+    # NOTE: On debian systems, nice always has an exit code of `0`, even when
+    # permission is denied.  Look for the error message instead.
+    if [ "$($APP_NICE_CMD true 2>&1)" != "" ]; then
         echo "ERROR: Permission denied to set application's niceness to" \
              "$APP_NICENESS.  Make sure the container is started with the" \
              "--cap-add=SYS_NICE option."