Don't use bash as the entrypoint for docker

The bash shell is not very good at playing init(1), let
tini handle this instead by doing `docker run --init`.

This makes it forward signals properly to e.g. make,
and also reaps processes for long-running containers.

Author: afbjorklund

Diff for: Makefile

@@ -114,7 +114,7 @@ HYPERKIT_LDFLAGS := -X k8s.io/minikube/pkg/drivers/hyperkit.version=$(VERSION) -
 
 # $(call DOCKER, image, command)
 define DOCKER
-	docker run --rm -e GOCACHE=/app/.cache -e IN_DOCKER=1 --user $(shell id -u):$(shell id -g) -w /app -v $(PWD):/app -v $(GOPATH):/go --entrypoint /bin/bash $(1) -c '$(2)'
+	docker run --rm -e GOCACHE=/app/.cache -e IN_DOCKER=1 --user $(shell id -u):$(shell id -g) -w /app -v $(PWD):/app -v $(GOPATH):/go --init $(1) /bin/bash -c '$(2)'
 endef
 
 ifeq ($(BUILD_IN_DOCKER),y)