fixes #3061: use variable for docker image name (#3062)

Makefile

@@ -12,6 +12,8 @@
 #   for pre-releases, we use   "1.0.0-beta.2" format
 VERSION=4.2.0-alpha.1
 
+DOCKER_IMAGE ?= quay.io/gravitational/teleport
+
 # These are standard autotools variables, don't change them please
 BUILDDIR ?= build
 BINDIR ?= /usr/local/bin
@@ -318,12 +320,12 @@ install: build
 .PHONY: image
 image:
 	cp ./build.assets/charts/Dockerfile $(BUILDDIR)/
-	cd $(BUILDDIR) && docker build --no-cache . -t quay.io/gravitational/teleport:$(VERSION)
+	cd $(BUILDDIR) && docker build --no-cache . -t $(DOCKER_IMAGE):$(VERSION)
 	if [ -f e/Makefile ]; then $(MAKE) -C e image; fi
 
 .PHONY: publish
 publish:
-	docker push quay.io/gravitational/teleport:$(VERSION)
+	docker push $(DOCKER_IMAGE):$(VERSION)
 	if [ -f e/Makefile ]; then $(MAKE) -C e publish; fi
 
 .PHONY: print-version