build: name the Valgrind run the container is prepared for

The Dockerfile installs valgrind and the ld.so debug info it needs, but
no target used them; docker-valgrind runs the unit suite under it, so
the dependency is visible and the run is one command.

Makefile

@@ -70,6 +70,10 @@ docker-check-stress:
 docker-bench:
 	$(DOCKER_RUN) sh -c 'make clean && make bench'
 
+docker-valgrind:
+	$(DOCKER_RUN) sh -c 'make clean && make -C tests unit_test && \
+		cd tests && valgrind -q --error-exitcode=1 ./unit_test $(UNITARGS)'
+
 docker: docker-image
 	$(MAKE) docker-build
 
@@ -95,4 +99,4 @@ verify-map:
 
 .PHONY: all check check-live check-stress test verify-map clean gtk bench \
 	docker docker-image docker-build docker-check docker-check-live \
-	docker-check-stress docker-bench
+	docker-check-stress docker-bench docker-valgrind