Fix make shell

The "shell" rule in the Makefile uses docker to run a bash session,
however it was depending on the "all" rule which assumes non-docker local
development. This commit fixes it by making it depend on the "runcimage" rule.

Signed-off-by: Tibor Vass <tibor@docker.com>

Makefile

@@ -82,7 +82,7 @@ rootlessintegration: runcimage
 localrootlessintegration: all
 	tests/rootless.sh
 
-shell: all
+shell: runcimage
 	docker run -e TESTFLAGS -ti --privileged --rm -v $(CURDIR):/go/src/$(PROJECT) $(RUNC_IMAGE) bash
 
 install: