Use '-s empty' instead of '-s dir'

This *should* have the same effect as the previous strategy: Instead of
'mkdir empty; fpm -s dir -C empty ...' we can simply do 'fpm -s empty'

Docker-DCO-1.1-Signed-off-by: Jordan Sissel <jls@semicomplete.com> (github: jordansissel)

hack/make/ubuntu

@@ -125,8 +125,7 @@ EOF
 		    --config-files /etc/default/docker \
 		    --deb-compression gz \
 		    -t deb .
-		mkdir empty
-		fpm -s dir -C empty \
+		fpm -s empty \
 		    --name lxc-docker --version $PKGVERSION \
 		    --architecture "$PACKAGE_ARCHITECTURE" \
 		    --depends lxc-docker-$VERSION \
@@ -135,7 +134,7 @@ EOF
 		    --url "$PACKAGE_URL" \
 		    --license "$PACKAGE_LICENSE" \
 		    --deb-compression gz \
-		    -t deb .
+		    -t deb
 	)
 }