Add circleci yaml for Dockerfile test build

Makefile

@@ -30,7 +30,7 @@ build: dependencies $(GOPATH)
 	$(GO) build -o prometheus $(BUILDFLAGS) github.com/prometheus/prometheus/cmd/prometheus
 	$(GO) build -o promtool $(BUILDFLAGS) github.com/prometheus/prometheus/cmd/promtool
 
-docker: build
+docker:
 	docker build -t prometheus:$(REV) .
 
 tarball: $(ARCHIVE)

README.md

@@ -1,4 +1,4 @@
-# Prometheus [![Build Status](https://travis-ci.org/prometheus/prometheus.svg)](https://travis-ci.org/prometheus/prometheus)
+# Prometheus [![Build Status](https://travis-ci.org/prometheus/prometheus.svg)](https://travis-ci.org/prometheus/prometheus) [![Circle CI](https://circleci.com/gh/prometheus/prometheus/tree/master.svg?style=svg)](https://circleci.com/gh/prometheus/prometheus/tree/master)
 
 Prometheus is a systems and service monitoring system. It collects metrics
 from configured targets at given intervals, evaluates rule expressions,

circle.yml

@@ -0,0 +1,11 @@
+machine:
+  services:
+    - docker
+
+dependencies:
+  override:
+    - make docker
+
+test:
+  override:
+    - /bin/true