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.

However, we _need_ to override the entrypoint of the
karalabe/xgo image, since it'll self-destruct otherwise.

It (/build.sh) expects the command to be the import path
and cannot handle getting the bash command line to run.

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)
@@ -427,7 +427,10 @@ out/minikube-installer.exe: out/minikube-windows-amd64.exe
 
 out/docker-machine-driver-hyperkit:
 ifeq ($(MINIKUBE_BUILD_IN_DOCKER),y)
-	$(call DOCKER,$(HYPERKIT_BUILD_IMAGE),CC=o64-clang CXX=o64-clang++ /usr/bin/make $@)
+	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 --entrypoint "" \
+		$(HYPERKIT_BUILD_IMAGE) /bin/bash -c 'CC=o64-clang CXX=o64-clang++ /usr/bin/make $@'
 else
 	GOOS=darwin CGO_ENABLED=1 go build \
 		-ldflags="$(HYPERKIT_LDFLAGS)"   \
@@ -436,7 +439,7 @@ endif
 
 hyperkit_in_docker:
 	rm -f out/docker-machine-driver-hyperkit
-	$(call DOCKER,$(HYPERKIT_BUILD_IMAGE),CC=o64-clang CXX=o64-clang++ /usr/bin/make out/docker-machine-driver-hyperkit)
+	$(MAKE) MINIKUBE_BUILD_IN_DOCKER=y out/docker-machine-driver-hyperkit
 
 .PHONY: install-hyperkit-driver
 install-hyperkit-driver: out/docker-machine-driver-hyperkit