dockerdeb: fix incorrect number of shifts

From the comment, it appears that the original intent was to remove the first
two arguments, so this patch does just that.

contrib/dockerdeb

@@ -11,7 +11,7 b' checkdocker'
 DISTID="$1"
 CODENAME="$2"
 PLATFORM="$1-$2"
-shift # extra params are passed to build process
+shift; shift # extra params are passed to build process
 
 OUTPUTDIR=${OUTPUTDIR:=$ROOTDIR/packages/$PLATFORM}