Allow dind image to be overriden.

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

README.md

@@ -9,6 +9,8 @@ A live version is available at: http://play-with-docker.com/
 
 ## Installation
 
+Start the Docker daemon on your machine and run `docker pull docker:1.12.2-rc2-dind`. 
+
 1) Install go 1.7.1 with `brew` on Mac or through a package manager.
 
 2) `go get`
@@ -21,5 +23,7 @@ A live version is available at: http://play-with-docker.com/
 
 Notes:
 
-There is a hard-coded limit to 5 Docker playgrounds per session. After 1 hour sessions are deleted.
+* There is a hard-coded limit to 5 Docker playgrounds per session. After 1 hour sessions are deleted.
+* If you want to override the DIND version or image then set the environmental variable i.e.
+  `DIND_IMAGE=docker:dind`
 

services/docker.go

@@ -2,7 +2,6 @@ package services
 
 import (
 	"log"
-	"os"
 	"strings"
 
 	ptypes "github.com/franela/play-with-docker/types"
@@ -75,18 +74,13 @@ func AttachExecConnection(execId string, ctx context.Context) (*types.HijackedRe
 	return &conn, nil
 }
 
-func CreateInstance(net string) (*ptypes.Instance, error) {
+func CreateInstance(net string, dindImage string) (*ptypes.Instance, error) {
 
 	var maximumPidLimit int64
 	maximumPidLimit = 150 // Set a ulimit value to prevent misuse
 	h := &container.HostConfig{NetworkMode: container.NetworkMode(net), Privileged: true}
 	h.Resources.PidsLimit = maximumPidLimit
 
-	dindImage := os.Getenv("DIND_IMAGE")
-	if len(dindImage) == 0 {
-		dindImage = "docker:1.12.2-rc2-dind"
-	}
-
 	conf := &container.Config{Image: dindImage}
 	container, err := c.ContainerCreate(context.Background(), conf, h, nil, "")
 

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