add tutorial for buildpacks run image override

codelab/03_buildpacks-runimage-override/README.md

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+### Example: buildpacks with custom run image (Go)
+
+This is an example demonstrating how to build a single Go file app with [Cloud Native Buildpacks](https://buildpacks.io/) using a custom [Run Image](https://buildpacks.io/docs/concepts/components/stack/) 
+
+[![Open in Cloud Shell](https://gstatic.com/cloudssh/images/open-btn.png)](https://ssh.cloud.google.com/cloudshell/open?cloudshell_git_repo=https://github.com/GoogleContainerTools/skaffold&cloudshell_working_dir=codelab/03_buildpacks-runimage-override&cloudshell_workspace=codelab/03_buildpacks-runimage-override&cloudshell_tutorial=tutorial.md)

codelab/03_buildpacks-runimage-override/app/go.mod

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+module github.com/GoogleContainerTools/skaffold/examples/buildpacks
+
+go 1.13

codelab/03_buildpacks-runimage-override/app/main.go

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+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+	"log"
+	"net/http"
+)
+
+func main() {
+	http.HandleFunc("/", hello)
+
+	log.Println("Listening on port 8080")
+	http.ListenAndServe(":8080", nil)
+}
+
+func hello(w http.ResponseWriter, _ *http.Request) {
+	data, err := ioutil.ReadFile("/hello.txt")
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+	}
+	fmt.Fprintf(w, string(data))
+}

codelab/03_buildpacks-runimage-override/base/Dockerfile

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+FROM gcr.io/gcp-runtimes/ubuntu_18_0_4
+LABEL "io.buildpacks.stack.id"="google"
+ENV CNB_STACK_ID io.buildpacks.stacks.bionic
+ENV CNB_USER_ID=1000
+ENV CNB_GROUP_ID=1000
+
+COPY hello.txt /

codelab/03_buildpacks-runimage-override/base/hello.txt

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+Hello, World!

codelab/03_buildpacks-runimage-override/k8s/web.yaml

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+apiVersion: v1
+kind: Service
+metadata:
+  name: web
+spec:
+  ports:
+  - port: 8080
+    name: http
+  type: LoadBalancer
+  selector:
+    app: web
+---
+apiVersion: apps/v1
+kind: Deployment
+metadata:
+  name: web
+spec:
+  selector:
+    matchLabels:
+      app: web
+  template:
+    metadata:
+      labels:
+        app: web
+    spec:
+      containers:
+      - name: web
+        image: app
+        ports:
+          - containerPort: 8080

codelab/03_buildpacks-runimage-override/skaffold.yaml

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+apiVersion: skaffold/v2beta9
+kind: Config
+build:
+  artifacts:
+  - image: base
+    context: base
+  - image: app
+    context: app
+    buildpacks:
+      builder: "gcr.io/buildpacks/builder:v1"
+      runImage: base
+      env:
+      - GOPROXY={{.GOPROXY}}
+    requires:
+    - image: base

codelab/03_buildpacks-runimage-override/tutorial.md

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+# Using custom run image for CNB builder
+
+## Introduction
+
+### What is CNB?
+[Cloud Native Buildpacks](https://buildpacks.io/) (CNB) enable building
+a container image from source code without the need for a Dockerfile.
+Skaffold supports building with CNB, requiring only
+a local Docker daemon. 
+
+CNB uses two images when building an application image:
+  - A _run image_ serves as the base image for the resulting application image.
+  - A _build image_ acts as the host for performing the build.
+
+### What you'll learn
+
+- how to annotate an arbitrary image and use it as a CNB 
+[run image](https://buildpacks.io/docs/concepts/components/stack/).
+- how to integrate the custom run image with a sample Buildpacks application using Skaffold's *artifact dependencies* feature.
+
+___
+
+**Time to complete**: <walkthrough-tutorial-duration duration=10></walkthrough-tutorial-duration>
+
+Click the **Start** button to move to the next step.
+
+## First steps
+
+### Start a Minikube cluster
+
+We'll use `minikube` as our local kubernetes cluster of choice.
+
+Run:
+```bash
+minikube start
+```
+
+## Write a custom CNB run image
+
+CNB run and build images require some additional metadata to identify the _Stack ID_ and user/group accounts to be used.  A [_stack_](https://buildpacks.io/docs/concepts/components/stack/) is a specification or contract.  For example, the `io.buildpacks.stacks.bionic` stack defines that it provides the same packages as installed on Ubuntu 18.04.
+
+We add a base artifact with a single <walkthrough-editor-open-file filePath="base/Dockerfile">Dockerfile</walkthrough-editor-open-file> that defines the required metadata, and reference this as an <walkthrough-editor-select-line filePath="skaffold.yaml" startLine="6" startCharacterOffset="4" endLine="8" endCharacterOffset="0">artifact</walkthrough-editor-select-line> called `base` in our `skaffold.yaml`.
+
+Next we'll use this artifact as the run image for a sample Buildpacks app.
+
+<walkthrough-footnote>
+    We will use the `gcr.io/buildpacks/builder:v1` builder image which supports the Stack ID `google`. So that's what we added to the Dockerfile.
+</walkthrough-footnote>
+
+## Use it in a sample Buildpacks app
+
+We use a simple <walkthrough-editor-open-file filePath="app/main.go">Go application</walkthrough-editor-open-file> and reference it as an <walkthrough-editor-select-line filePath="skaffold.yaml" startLine="6" startCharacterOffset="4" endLine="8" endCharacterOffset="0">artifact</walkthrough-editor-select-line> called `app` in our `skaffold.yaml`. 
+
+To use the `base` artifact as the custom run image we:
+- add an <walkthrough-editor-select-line filePath="skaffold.yaml" startLine="13" startCharacterOffset="4" endLine="15" endCharacterOffset="0">artifact dependency</walkthrough-editor-select-line> for `app` artifact on the `base` artifact.
+- set the <walkthrough-editor-select-line filePath="skaffold.yaml" startLine="10" startCharacterOffset="6" endLine="11" endCharacterOffset="0">runImage</walkthrough-editor-select-line> property of the `app` artifact to be the `base` artifact.
+
+## Run it!
+
+Run this command and Skaffold should take care of building the artifacts in order and deploying the provided <walkthrough-editor-open-file filePath="k8s/web.yaml">manifest</walkthrough-editor-open-file>.
+
+```bash
+skaffold dev --port-forward
+```
+
+Once the image has been built and deployed click on the <walkthrough-web-preview-icon></walkthrough-web-preview-icon> icon and select `Preview on port 8080`. This should redirect to the running service and show the output:
+
+```
+Hello, World!
+```
+
+The Go app reads <walkthrough-editor-open-file filePath="base/hello.txt">hello.txt</walkthrough-editor-open-file> that's provided by the base artifact. Lets change the text from <walkthrough-editor-select-line filePath="base/hello.txt" startLine="0" startCharacterOffset="0" endLine="1" endCharacterOffset="0">Hello, World!</walkthrough-editor-select-line> to `Hello, Buildpacks!`. This should trigger a rebuild of the `base` artifact which in turn triggers a rebuild and redeploy for the `app` artifact. Once that completes click on the <walkthrough-web-preview-icon></walkthrough-web-preview-icon> icon again and select `Preview on port 8080`. This should redirect to the running service and show the output:
+
+```
+Hello, Buildpacks!
+```
+
+## Congratulations
+
+<walkthrough-conclusion-trophy></walkthrough-conclusion-trophy>
+
+All done!
+
+You now know how to use Buildpacks with custom run images and use Skaffold to tie the loop together.
+
+