Makefile: Introduce makefile to ease testing

Refs. swh/infra/sysadm-environment#4794

Makefile

+IMAGE="helmunittest/helm-unittest:3.11.1-0.3.0"
+# requires --user in the docker call to allow writing in the user's home
+UID=1000
+
+test:
+	docker run -ti --user $(UID) --rm -v $(PWD):/apps $(IMAGE) swh