Enable embedding of ko publish (#348)

- Export functions and a variable to enable embedding of ko's
  `publish` functionality to be embedded in other tools.

  See https://github.com/GoogleContainerTools/skaffold/pull/5611

- Remove DockerRepo PublishOption and flag.

  This removes the `DockerRepo` config option and `--docker-repo`
  flag from the PR.

  New PR with the extracted config option:
  https://github.com/google/ko/pull/351

- Fix copyright headers for boilerplate check.

- Use DockerRepo PublishOption instead of env var.

- Override defaultBaseImage using BuildOptions.

  Remove exported package global SetDefaultBaseImage and instead
  allow programmatic override of the default base image using
  the field `BaseImage` in `options.BuildOptions`.

  Also fix copyright header years.

- Add BuildOptions parameter to getBaseImage

  This enables access to BaseImage for programmatically overriding
  the default base image from `.ko.yaml`.

- Add UserAgent to BuildOptions and PublishOptions

  This enables programmatically overriding the `User-Agent` HTTP
  request header for both pulling the base image and pushing the
  built image.

- Rename MakeBuilder to NewBuilder and MakePublisher to NewPublisher.

  For more idiomatic constructor function names.

pkg/commands/config_test.go

@@ -0,0 +1,47 @@
+/*
+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.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package commands
+
+import (
+	"context"
+	"testing"
+
+	"github.com/google/ko/pkg/commands/options"
+)
+
+func TestOverrideDefaultBaseImageUsingBuildOption(t *testing.T) {
+	wantDigest := "sha256:76c39a6f76890f8f8b026f89e081084bc8c64167d74e6c93da7a053cb4ccb5dd"
+	wantImage := "gcr.io/distroless/static-debian9@" + wantDigest
+	bo := &options.BuildOptions{
+		BaseImage: wantImage,
+	}
+
+	baseFn := getBaseImage("all", bo)
+	res, err := baseFn(context.Background(), "ko://example.com/helloworld")
+	if err != nil {
+		t.Fatalf("getBaseImage(): %v", err)
+	}
+
+	digest, err := res.Digest()
+	if err != nil {
+		t.Fatalf("res.Digest(): %v", err)
+	}
+	gotDigest := digest.String()
+	if gotDigest != wantDigest {
+		t.Errorf("got digest %s, wanted %s", gotDigest, wantDigest)
+	}
+}