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/pull_artifact.go

@@ -0,0 +1,81 @@
+/*
+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 (
+	"context"
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+
+	"github.com/fluxcd/flux2/internal/oci"
+)
+
+var pullArtifactCmd = &cobra.Command{
+	Use:   "artifact",
+	Short: "Push artifact",
+	Long: `The pull artifact command downloads and extracts the OCI artifact content to the given path.
+The pull command uses the credentials from '~/.docker/config.json'.`,
+	Example: `# Pull an OCI artifact created by flux from GHCR
+flux pull artifact ghcr.io/org/manifests/app:v0.0.1 --output ./path/to/local/manifests
+`,
+	RunE: pullArtifactCmdRun,
+}
+
+type pullArtifactFlags struct {
+	output string
+}
+
+var pullArtifactArgs pullArtifactFlags
+
+func init() {
+	pullArtifactCmd.Flags().StringVarP(&pullArtifactArgs.output, "output", "o", "", "Path where the artifact content should be extracted.")
+	pullCmd.AddCommand(pullArtifactCmd)
+}
+
+func pullArtifactCmdRun(cmd *cobra.Command, args []string) error {
+	if len(args) < 1 {
+		return fmt.Errorf("artifact name is required")
+	}
+	url := args[0]
+
+	if pullArtifactArgs.output == "" {
+		return fmt.Errorf("invalid output path %s", pullArtifactArgs.output)
+	}
+
+	if fs, err := os.Stat(pullArtifactArgs.output); err != nil || !fs.IsDir() {
+		return fmt.Errorf("invalid output path %s", pullArtifactArgs.output)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), rootArgs.timeout)
+	defer cancel()
+
+	logger.Actionf("pulling artifact from %s", url)
+
+	meta, err := oci.Pull(ctx, url, pullArtifactArgs.output)
+	if err != nil {
+		return err
+	}
+
+	logger.Successf("source %s", meta.Source)
+	logger.Successf("revision %s", meta.Revision)
+	logger.Successf("digest %s", meta.Digest)
+	logger.Successf("artifact content extracted to %s", pullArtifactArgs.output)
+
+	return nil
+}