use newer docker and login format

Dockerfile

@@ -1,4 +1,4 @@
-FROM gliderlabs/alpine:3.2
+FROM gliderlabs/alpine:3.3
 
 RUN apk-install docker git go haproxy python
 

api/cmd/build/main.go

@@ -82,10 +82,14 @@ func main() {
 		die(err)
 	}
 
-	cfg := []byte(*dockercfg)
+	if *dockercfg != "" {
+		err := os.MkdirAll("/root/.docker", 0700)
 
-	if len(cfg) > 0 {
-		err = ioutil.WriteFile("/root/.dockercfg", cfg, 0400)
+		if err != nil {
+			die(err)
+		}
+
+		err = ioutil.WriteFile("/root/.docker/config.json", []byte(*dockercfg), 0400)
 
 		if err != nil {
 			die(err)

api/models/build.go

@@ -244,7 +244,7 @@ func (b *Build) buildArgs(cache bool, config string) ([]string, error) {
 		args = append(args, "-no-cache")
 	}
 
-	if dockercfg, err := ioutil.ReadFile("/root/.dockercfg"); err == nil {
+	if dockercfg, err := ioutil.ReadFile("/root/.docker/config.json"); err == nil {
 		args = append(args, "-dockercfg", string(dockercfg))
 	}