Allow dind image to be overriden.

- improve readme - mention pre-pulling dind image.
- read env variable once in init() method

services/instance.go

@@ -1,19 +1,37 @@
 package services
 
-import "github.com/franela/play-with-docker/types"
+import (
+	"log"
+	"os"
+
+	"github.com/franela/play-with-docker/types"
+)
 
 var instances map[string]map[string]*types.Instance
 
+var dindImage string
+var defaultDindImageName string
+
 func init() {
 	instances = make(map[string]map[string]*types.Instance)
+        dindImage = getDindImageName()
+}
+
+func getDindImageName() string {
+	dindImage := os.Getenv("DIND_IMAGE")
+        defaultDindImageName = "docker:1.12.2-rc2-dind"
+        if len(dindImage) == 0 {
+		dindImage = defaultDindImageName
+	}
+	return dindImage
 }
 
 func NewInstance(session *types.Session) (*types.Instance, error) {
-	//TODO: Validate that a session can only have 10 instances
 
+	//TODO: Validate that a session can only have 5 instances
 	//TODO: Create in redis
-
-	instance, err := CreateInstance(session.Id)
+	log.Printf("NewInstance - using image: [%s]\n", dindImage)
+	instance, err := CreateInstance(session.Id, dindImage)
 
 	if err != nil {
 		return nil, err