Resolve issue #356

Improve logging for the prepare step

Before
```
$ logs -image tutorial-image -build 1
[prepare] prepare:fetch.go:88: Successfully cloned "https://github.com/spring-projects/spring-petclinic" @ "82cb521d636b282340378d80a6307a08e3d4a4c4" in path "/workspace"
```

After:
```
$ logs -image tutorial-image -build 5
[prepare] Loading secrets for "https://index.docker.io/v1/" from secret "tutorial-registry-credentials"
[prepare] Successfully cloned "https://github.com/spring-projects/spring-petclinic" @ "400e3028f48a6c23f5156f6598dd10cb5e6a2849" in path "/workspace"
```

Signed-off-by: Sukhil Suresh <ssuresh@pivotal.io>

cmd/build-init/main.go

@@ -55,9 +55,9 @@ const (
 func main() {
 	flag.Parse()
 
-	logger := log.New(os.Stdout, "prepare:", log.Lshortfile)
+	logger := log.New(os.Stdout, "", 0)
 
-	creds, err := dockercreds.ParseMountedAnnotatedSecrets(buildSecretsDir, dockerCredentials)
+	creds, err := dockercreds.ParseMountedAnnotatedSecrets(buildSecretsDir, dockerCredentials, logger)
 	if err != nil {
 		logger.Fatal(err)
 	}

cmd/rebase/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"flag"
+	"log"
 	"os"
 
 	"github.com/buildpacks/imgutil/remote"
@@ -34,16 +35,17 @@ func init() {
 func main() {
 	flag.Parse()
 	tags := flag.Args()
+	logger := log.New(os.Stdout, "", 0)
 
-	cmd.Exit(rebase(tags))
+	cmd.Exit(rebase(tags, logger))
 }
 
-func rebase(tags []string) error {
+func rebase(tags []string, logger *log.Logger) error {
 	if len(tags) < 1 {
 		return cmd.FailCode(cmd.CodeInvalidArgs, "must provide one or more image tags")
 	}
 
-	keychain, err := dockercreds.ParseMountedAnnotatedSecrets(buildSecretsDir, dockerCredentials)
+	keychain, err := dockercreds.ParseMountedAnnotatedSecrets(buildSecretsDir, dockerCredentials, logger)
 	if err != nil {
 		return cmd.FailErrCode(err, cmd.CodeInvalidArgs)
 	}

pkg/dockercreds/parse_annoted_secrets.go

@@ -1,14 +1,16 @@
 package dockercreds
 
 import (
+	"log"
 	"strings"
 
 	"github.com/google/go-containerregistry/pkg/authn"
-	"github.com/pivotal/kpack/pkg/secret"
 	"github.com/pkg/errors"
+
+	"github.com/pivotal/kpack/pkg/secret"
 )
 
-func ParseMountedAnnotatedSecrets(volumeName string, secrets []string) (DockerCreds, error) {
+func ParseMountedAnnotatedSecrets(volumeName string, secrets []string, logger *log.Logger) (DockerCreds, error) {
 	var dockerCreds = DockerCreds{}
 	for _, s := range secrets {
 		splitSecret := strings.Split(s, "=")
@@ -18,6 +20,7 @@ func ParseMountedAnnotatedSecrets(volumeName string, secrets []string) (DockerCr
 		secretName := splitSecret[0]
 		domain := splitSecret[1]
 
+		logger.Printf("Loading secrets for %q from secret %q", domain, secretName)
 		auth, err := secret.ReadBasicAuthSecret(volumeName, secretName)
 		if err != nil {
 			return nil, err

pkg/dockercreds/parse_annoted_secrets_test.go

@@ -1,7 +1,9 @@
 package dockercreds_test
 
 import (
+	"bytes"
 	"io/ioutil"
+	"log"
 	"os"
 	"path"
 	"testing"
@@ -44,10 +46,16 @@ func testParseAnnotatedSecrets(t *testing.T, when spec.G, it spec.S) {
 
 		when("ParseMountedAnnotatedSecrets", func() {
 			it("parses the volume mounted creds", func() {
+
+				logger := log.New(&bytes.Buffer{}, "", 0)
+
 				creds, err := dockercreds.ParseMountedAnnotatedSecrets(testDir,
 					[]string{
 						"gcr-creds=gcr.io",
-						"dockerhub-creds=index.docker.io"})
+						"dockerhub-creds=index.docker.io",
+					},
+					logger,
+				)
 				require.NoError(t, err)
 
 				assert.Equal(t, dockercreds.DockerCreds{