Fix registry URL regexp

Sanity checking was a little too strict. This is better and should allow
for default config of gitlab.com, which doesn't include port numbers.

makefile-sanity.include

@@ -1,6 +1,6 @@
 ifdef DOCKER_REGISTRY
-ifneq ($(DOCKER_REGISTRY), $(shell echo $(DOCKER_REGISTRY) | sed -ne '/^[A-Za-z0-9.\/\-]\+:[0-9]\+\([A-Za-z0-9.\/-]\+\)\?$$/p'))
-$(error Bad docker registry URL. Should follow format registry.example.com:1234 or registry.example.com:1234/foo)
+ifneq ($(DOCKER_REGISTRY), $(shell echo $(DOCKER_REGISTRY) | sed -ne '/^[A-Za-z0-9.\/\-]\+\(:[0-9]\+\)\?\([A-Za-z0-9.\/-]\+\)\?$$/p'))
+$(error Bad docker registry URL. Should follow format registry.example.com/foo, registry.example.com:1234 or registry.example.com:1234/foo)
 endif
     REGISTRY=$(DOCKER_REGISTRY)/
 else