[docker] Fix some issues in the Makefile for the docker image.

docker/Makefile

@@ -25,14 +25,14 @@
 all: Dockerfile
 	@echo "You may want to consider adding ARGS=--no-cache to force"
 	@echo "Docker to rebuild all layers."
-	docker build . -t laiser/caisar:pub -f $^ --rm
+	docker build . -t laiser/caisar:pub -f $^ --rm $(ARGS)
 
 push: all
 	@echo "You may need to execute 'docker login' beforehand."
 	docker push laiser/caisar:pub
 
 Dockerfile: Makefile Dockerfile.template env.template
-	sed 's|@UBUNTU_OPAM_BASE@|ocaml/opam:ubuntu-ocaml-4.13|g' Dockerfile.template | \
+	sed 's|@UBUNTU_OPAM_BASE@|ocaml/opam:ubuntu-ocaml-@OCAMLV@|g' Dockerfile.template | \
 sed 's|@ENV@|$(shell cat env.template)|g' | \
 sed 's|@OPAM_DEPS@|conf-protoc conf-gmp conf-autoconf conf-pkg-config|' | \
 sed 's|@OCAMLV@|4.13|g' | \