Add artifact commands

Implement build, push, pull and tag artifact commands.
For authentication purposes, all `flux <verb> artifact` commands are using the '~/.docker/config.json' config file and the Docker credential helpers.

Signed-off-by: Stefan Prodan <stefan.prodan@gmail.com>

cmd/flux/build_artifact.go

@@ -0,0 +1,69 @@
+/*
+Copyright 2022 The Flux authors
+
+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 main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+
+	"github.com/fluxcd/flux2/internal/oci"
+)
+
+var buildArtifactCmd = &cobra.Command{
+	Use:   "artifact",
+	Short: "Build artifact",
+	Long:  `The build artifact command creates an tgz file with the manifests from the given directory.`,
+	Example: `# Build the given manifests directory into an artifact
+flux build artifact --path ./path/to/local/manifests --output ./path/to/artifact.tgz
+`,
+	RunE: buildArtifactCmdRun,
+}
+
+type buildArtifactFlags struct {
+	output string
+	path   string
+}
+
+var buildArtifactArgs buildArtifactFlags
+
+func init() {
+	buildArtifactCmd.Flags().StringVar(&buildArtifactArgs.path, "path", "", "Path to the directory where the Kubernetes manifests are located.")
+	buildArtifactCmd.Flags().StringVarP(&buildArtifactArgs.output, "output", "0", "artifact.tgz", "Path to where the artifact tgz file should be written.")
+	buildCmd.AddCommand(buildArtifactCmd)
+}
+
+func buildArtifactCmdRun(cmd *cobra.Command, args []string) error {
+	if buildArtifactArgs.path == "" {
+		return fmt.Errorf("invalid path %q", buildArtifactArgs.path)
+	}
+
+	if fs, err := os.Stat(buildArtifactArgs.path); err != nil || !fs.IsDir() {
+		return fmt.Errorf("invalid path %q", buildArtifactArgs.path)
+	}
+
+	logger.Actionf("building artifact from %s", buildArtifactArgs.path)
+
+	if err := oci.Build(buildArtifactArgs.output, buildArtifactArgs.path); err != nil {
+		return fmt.Errorf("bulding artifact failed, error: %w", err)
+	}
+
+	logger.Successf("artifact created at %s", buildArtifactArgs.output)
+
+	return nil
+}