Allow docker engine to be pulled from s3 bucket

Use aws cli to pull docker engine binaries from s3 bucket.  Conditional and
happens if DOCKER_BIN_URL begins with 's3://'

Example usage:
```
  export AWS_ACCESS_KEY_ID=key
  export AWS_SECRET_ACCESS_KEY=secret
  make DOCKER_BIN_URL=s3://path-to-engine-binaries
```
Signed-off-by: Robb Kistler <robb.kistler@docker.com>

Signed-off-by: Robb Kistler <robb.kistler@docker.com>

tools/aws-cli/Dockerfile

@@ -0,0 +1,11 @@
+FROM alpine:3.5
+
+RUN \
+	mkdir -p /aws && \
+	apk -Uuv add groff less python py-pip && \
+	pip install awscli && \
+	apk --purge -v del py-pip && \
+	rm /var/cache/apk/*
+
+WORKDIR /aws
+ENTRYPOINT ["aws"]

tools/aws-cli/Makefile

@@ -0,0 +1,29 @@
+.PHONY: tag push
+
+BASE=alpine:3.5
+IMAGE=aws-cli
+
+default: push
+
+hash: Dockerfile
+	DOCKER_CONTENT_TRUST=1 docker pull $(BASE)
+	tar cf - $^ | docker build --no-cache -t $(IMAGE):build -
+	docker run --rm --entrypoint=/bin/sh $(IMAGE):build -c 'cat /Dockerfile /lib/apk/db/installed | sha1sum' | sed 's/ .*//' > hash
+
+push: hash
+	docker pull mobylinux/$(IMAGE):$(shell cat hash) || \
+		(docker tag $(IMAGE):build mobylinux/$(IMAGE):$(shell cat hash) && \
+		 docker push mobylinux/$(IMAGE):$(shell cat hash))
+	docker rmi $(IMAGE):build
+	rm -f hash
+
+tag: hash
+	docker pull mobylinux/$(IMAGE):$(shell cat hash) || \
+		docker tag $(IMAGE):build mobylinux/$(IMAGE):$(shell cat hash)
+	docker rmi $(IMAGE):build
+	rm -f hash
+
+clean:
+	rm -f hash
+
+.DELETE_ON_ERROR: