dockerdeb: redirect 'cd' in export command to /dev/null

This had the unfortunate side effect of causing the environment to have a
newline due to the fact that some 'cd' outputs the result of the directory
change. So, let's just redirect the meaningless output.

contrib/dockerdeb

@@ -4,7 +4,7 b''
 . $(dirname $0)/packagelib.sh
 
 BUILDDIR=$(dirname $0)
-export ROOTDIR=$(cd $BUILDDIR/..; pwd)
+export ROOTDIR=$(cd $BUILDDIR/.. > /dev/null; pwd)
 
 checkdocker