Add flag and PublishOption for destination repo

This enables programmatically setting the destination image repository
when embedding ko's `publish` functionality in other tools.

See ko-build#348

README.md

@@ -61,12 +61,16 @@ to
 
 ## Choose Destination
 
-`ko` depends on an environment variable, `KO_DOCKER_REPO`, to identify where it
-should push images that it builds. Typically this will be a remote registry,
-e.g.:
+`ko` must be configured with a destination for where it should push images that
+it builds. You can configure this with either the `--docker-repo` flag or the
+`KO_DOCKER_REPO` environment variable. Typically, the value will be a remote
+registry, e.g.:
 
 - `KO_DOCKER_REPO=gcr.io/my-project`, or
-- `KO_DOCKER_REPO=my-dockerhub-user`
+- `--docker-repo=my-dockerhub-user`
+
+If both the flag and the environment variable are set, the flag value takes
+precedence.
 
 # Build an Image
 

pkg/commands/options/publish.go

@@ -1,4 +1,4 @@
-// Copyright 2018 Google LLC All Rights Reserved.
+// Copyright 2021 Google LLC All Rights Reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -25,6 +25,9 @@ import (
 
 // PublishOptions encapsulates options when publishing.
 type PublishOptions struct {
+	// DockerRepo overrides the KO_DOCKER_REPO environment variable, if present
+	DockerRepo string
+
 	Tags []string
 
 	// Push publishes images to a registry.
@@ -46,6 +49,8 @@ type PublishOptions struct {
 }
 
 func AddPublishArg(cmd *cobra.Command, po *PublishOptions) {
+	cmd.Flags().StringVar(&po.DockerRepo, "docker-repo", "", "Repository to push images, overrides KO_DOCKER_REPO")
+
 	cmd.Flags().StringSliceVarP(&po.Tags, "tags", "t", []string{"latest"},
 		"Which tags to use for the produced image instead of the default 'latest' tag "+
 			"(may not work properly with --base-import-paths or --bare).")

pkg/commands/resolver.go

@@ -1,4 +1,4 @@
-// Copyright 2018 Google LLC All Rights Reserved.
+// Copyright 2021 Google LLC All Rights Reserved.
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -132,6 +132,9 @@ func makePublisher(po *options.PublishOptions) (publish.Interface, error) {
 	// to either a docker daemon or a container image registry.
 	innerPublisher, err := func() (publish.Interface, error) {
 		repoName := os.Getenv("KO_DOCKER_REPO")
+		if po.DockerRepo != "" {
+			repoName = po.DockerRepo
+		}
 		namer := options.MakeNamer(po)
 		if repoName == publish.LocalDomain || po.Local {
 			// TODO(jonjohnsonjr): I'm assuming that nobody will
@@ -144,7 +147,7 @@ func makePublisher(po *options.PublishOptions) (publish.Interface, error) {
 		}
 
 		if repoName == "" {
-			return nil, errors.New("KO_DOCKER_REPO environment variable is unset")
+			return nil, errors.New("either --docker-repo flag or KO_DOCKER_REPO environment variable must be set")
 		}
 		if _, err := name.NewRegistry(repoName); err != nil {
 			if _, err := name.NewRepository(repoName); err != nil {