Drops privileges to user 101 and group 0

When the container is started as root the default was to drop privileges to "unit:unit". This caused some problems with temporary files. Now the privileges are drop to "101:0".
When the container is started as a normal user unit prints a warning that changing the user and group is not possible. This warning is safe to ignore.

Author: tobiasge

docker/launch-netbox.sh

@@ -51,4 +51,6 @@ exec unitd \
   --pid /opt/unit/unit.pid \
   --log /dev/stdout \
   --state /opt/unit/state/ \
-  --tmp /opt/unit/tmp/
+  --tmp /opt/unit/tmp/ \
+  --user 101 \
+  --group 0