update the default location of output images

- fix some docs and comments

cmd/imagedigestexporter/main.go

@@ -24,7 +24,6 @@ import (
 
 	"github.com/google/go-containerregistry/pkg/v1/layout"
 	v1alpha1 "github.com/tektoncd/pipeline/pkg/apis/pipeline/v1alpha1"
-	_ "k8s.io/client-go/plugin/pkg/client/auth/gcp"
 )
 
 var (

docs/developers/README.md

@@ -146,3 +146,10 @@ with the binary and file(s) is mounted.
 
 If the image is a private registry, the service account should include an
 [ImagePullSecret](https://kubernetes.io/docs/tasks/configure-pod-container/configure-service-account/#add-imagepullsecrets-to-a-service-account)
+
+## Builder namespace on containers
+
+The `/builder/` namespace is reserved on containers for various system tools, such as the following:
+
+ - The environment variable HOME is set to `/builder/home`, used by the builder tools and injected on into all of the step containers
+ - Default location for output-images `/builder/output-images` 

docs/resources.md

@@ -184,7 +184,7 @@ spec:
       outputImageDir: /workspace/builtImage
   steps: ...
 ```
-If no value is specified for `outputImageDir`, it will default to `/tools/image-outputs/{resource-name}`.
+If no value is specified for `outputImageDir`, it will default to `/builder/image-outputs/{resource-name}`.
 
 *Please check the builder tool used on how to pass this path to create the output file.*  
 

pkg/apis/pipeline/v1alpha1/image_resource.go

@@ -98,8 +98,8 @@ func (s *ImageResource) GetOutputImageDir() string {
 }
 
 func (s ImageResource) String() string {
-	// the String() func works as a toString func to return the contents as a string
-	// and has to follow the interface and therefore cannot return an error
+	// the String() func implements the Stringer interface, and therefore
+	// cannot return an error
 	// if the Marshal func gives an error, the returned string will be empty
 	json, _ := json.Marshal(s)
 	return string(json)

pkg/apis/pipeline/v1alpha1/task_defaults.go

@@ -31,7 +31,7 @@ func (ts *TaskSpec) SetDefaults(ctx context.Context) {
 		for i, o := range ts.Outputs.Resources {
 			if o.Type == PipelineResourceTypeImage {
 				if o.OutputImageDir == "" {
-					ts.Outputs.Resources[i].OutputImageDir = fmt.Sprintf("/tools/image-outputs/%s", o.Name)
+					ts.Outputs.Resources[i].OutputImageDir = fmt.Sprintf("/builder/image-outputs/%s", o.Name)
 				}
 			}
 		}