Add ability to set git auth token using environment variables

Currently the only way to set the git auth is by including it in the context
git URL. This can be problematic for certain environments such as ECS where the
command args for are stored in logs. Introduces updates to allow setting the
authentication info using the environment variables `GIT_USERNAME` and
`GIT_PASSWORD`.

pkg/buildcontext/git.go

@@ -23,12 +23,17 @@ import (
 	"github.com/GoogleContainerTools/kaniko/pkg/constants"
 	git "gopkg.in/src-d/go-git.v4"
 	"gopkg.in/src-d/go-git.v4/plumbing"
+	"gopkg.in/src-d/go-git.v4/plumbing/transport"
+	"gopkg.in/src-d/go-git.v4/plumbing/transport/http"
 )
 
 const (
 	gitPullMethodEnvKey = "GIT_PULL_METHOD"
 	gitPullMethodHTTPS  = "https"
 	gitPullMethodHTTP   = "http"
+
+	gitAuthUsernameEnvKey = "GIT_USERNAME"
+	gitAuthPasswordEnvKey = "GIT_PASSWORD"
 )
 
 var (
@@ -46,6 +51,7 @@ func (g *Git) UnpackTarFromBuildContext() (string, error) {
 	parts := strings.Split(g.context, "#")
 	options := git.CloneOptions{
 		URL:      getGitPullMethod() + "://" + parts[0],
+		Auth:     getGitAuth(),
 		Progress: os.Stdout,
 	}
 	if len(parts) > 1 {
@@ -55,6 +61,18 @@ func (g *Git) UnpackTarFromBuildContext() (string, error) {
 	return directory, err
 }
 
+func getGitAuth() transport.AuthMethod {
+	username := os.Getenv(gitAuthUsernameEnvKey)
+	password := os.Getenv(gitAuthPasswordEnvKey)
+	if username != "" || password != "" {
+		return &http.BasicAuth{
+			Username: username,
+			Password: password,
+		}
+	}
+	return nil
+}
+
 func getGitPullMethod() string {
 	gitPullMethod := os.Getenv(gitPullMethodEnvKey)
 	if ok := supportedGitPullMethods[gitPullMethod]; !ok {