Refactor command line arguments and the executor

In this refactor I:

1. Created KanikoOptions to make it easier to pass around arguments
passed in through the command line
2. Reorganized executor.go by putting the logic for pushing the image in
a new file push.go
3. Made some error messages clearer
4. Fixed a mistake in the README for pushing to AWS
5. Marked the --bucket flag as hidden since we want people to use
--context instead, and marked an aws flag as hidden which is set in a
vendored directorya

pkg/executor/push.go

@@ -0,0 +1,95 @@
+/*
+Copyright 2018 Google LLC
+
+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 executor
+
+import (
+	"crypto/tls"
+	"fmt"
+	"net/http"
+
+	"github.com/GoogleContainerTools/kaniko/pkg/options"
+	"github.com/GoogleContainerTools/kaniko/pkg/version"
+	"github.com/google/go-containerregistry/pkg/authn"
+	"github.com/google/go-containerregistry/pkg/authn/k8schain"
+	"github.com/google/go-containerregistry/pkg/name"
+	"github.com/google/go-containerregistry/pkg/v1"
+	"github.com/google/go-containerregistry/pkg/v1/remote"
+	"github.com/google/go-containerregistry/pkg/v1/tarball"
+	"github.com/sirupsen/logrus"
+)
+
+type withUserAgent struct {
+	t http.RoundTripper
+}
+
+func (w *withUserAgent) RoundTrip(r *http.Request) (*http.Response, error) {
+	r.Header.Set("User-Agent", fmt.Sprintf("kaniko/%s", version.Version()))
+	return w.t.RoundTrip(r)
+}
+
+// DoPush is responsible for pushing image to the destinations specified in opts
+func DoPush(image v1.Image, opts *options.KanikoOptions) error {
+	if opts.NoPush {
+		logrus.Info("Skipping push to container registry due to --no-push flag")
+		return nil
+	}
+	// continue pushing unless an error occurs
+	for _, destination := range opts.Destinations {
+		// Push the image
+		destRef, err := name.NewTag(destination, name.WeakValidation)
+		if err != nil {
+			return err
+		}
+
+		if opts.DockerInsecureSkipTLSVerify {
+			newReg, err := name.NewInsecureRegistry(destRef.Repository.Registry.Name(), name.WeakValidation)
+			if err != nil {
+				return err
+			}
+			destRef.Repository.Registry = newReg
+		}
+
+		if opts.TarPath != "" {
+			return tarball.WriteToFile(opts.TarPath, destRef, image, nil)
+		}
+
+		k8sc, err := k8schain.NewNoClient()
+		if err != nil {
+			return err
+		}
+		kc := authn.NewMultiKeychain(authn.DefaultKeychain, k8sc)
+		pushAuth, err := kc.Resolve(destRef.Context().Registry)
+		if err != nil {
+			return err
+		}
+
+		// Create a transport to set our user-agent.
+		tr := http.DefaultTransport
+		if opts.DockerInsecureSkipTLSVerify {
+			tr.(*http.Transport).TLSClientConfig = &tls.Config{
+				InsecureSkipVerify: true,
+			}
+		}
+		rt := &withUserAgent{t: tr}
+
+		if err := remote.Write(destRef, image, pushAuth, rt, remote.WriteOptions{}); err != nil {
+			logrus.Error(fmt.Errorf("Failed to push to destination %s", destination))
+			return err
+		}
+	}
+	return nil
+}