Add GitLab CI credentials helper (#2040)

This simplifies usage of Kaniko in GitLab CI environments and
means that it's not longer necessary to manually cobble together
the config with the right values from the environment in
error-prone shell commands.

pkg/creds/creds.go

@@ -21,6 +21,7 @@ import (
 
 	ecr "github.com/awslabs/amazon-ecr-credential-helper/ecr-login"
 	"github.com/chrismellard/docker-credential-acr-env/pkg/credhelper"
+	gitlab "github.com/ePirat/docker-credential-gitlabci/pkg/credhelper"
 	"github.com/google/go-containerregistry/pkg/authn"
 	"github.com/google/go-containerregistry/pkg/v1/google"
 )
@@ -32,5 +33,6 @@ func GetKeychain() authn.Keychain {
 		google.Keychain,
 		authn.NewKeychainFromHelper(ecr.NewECRHelper(ecr.WithLogger(ioutil.Discard))),
 		authn.NewKeychainFromHelper(credhelper.NewACRCredentialsHelper()),
+		authn.NewKeychainFromHelper(gitlab.NewGitLabCredentialsHelper()),
 	)
 }