Run make tag on pkg directory to check packages build

This at least checks for buildability of packages, even if we
are not yet pushing them.

See https://github.com/linuxkit/linuxkit/issues/1991 for what it mitigates.
Will not pass until this is fixed.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

pkg/Makefile

@@ -1,10 +1,10 @@
 DIRS = $(shell find . -type d -depth 1)
 .PHONY: clean dirs $(DIRS)
 
-push: $(DIRS)
+push:
+	@set -e; for d in $(DIRS); do make -C "$$d"; done
 
-$(DIRS):
-	$(MAKE) -C $@
+tag:
+	@set -e; for d in $(DIRS); do make -C "$$d" tag; done
 
-clean:
-	rm -f hash
+clean: ;