Make CI script to verify that vendor is in sync

Signed-off-by: Odin Ugedal <odin@ugedal.com>

Makefile

@@ -1,7 +1,7 @@
 .PHONY: all shell dbuild man release \
 	    localtest localunittest localintegration \
 	    test unittest integration \
-	    cross localcross
+	    cross localcross vendor vendor-verify
 
 GO := go
 
@@ -116,7 +116,16 @@ validate:
 	script/validate-c
 	$(GO) vet $(allpackages)
 
-ci: validate test release
+ci: validate test vendor-verify release
+
+vendor:
+	export GO111MODULE=on \
+	$(GO) mod tidy && \
+	$(GO) mod vendor && \
+	$(GO) mod verify
+
+vendor-verify: vendor
+	git diff --exit-code -- go.mod go.sum vendor/ || (echo 'error: vendor/ and/or go.mod not up to date. Run "make vendor" to update'; exit 1)
 
 cross: runcimage
 	docker run ${DOCKER_RUN_PROXY} -e BUILDTAGS="$(BUILDTAGS)" --rm -v $(CURDIR):/go/src/$(PROJECT) $(RUNC_IMAGE) make localcross