Add global --override-arch and --override-os options

This e.g. allows accessing Linux images on macOS.

cmd/skopeo/layers.go

@@ -24,11 +24,15 @@ var layersCmd = cli.Command{
 		if c.NArg() == 0 {
 			return errors.New("Usage: layers imageReference [layer...]")
 		}
+		ctx, err := contextFromGlobalOptions(c, "")
+		if err != nil {
+			return err
+		}
 		rawSource, err := parseImageSource(c, c.Args()[0])
 		if err != nil {
 			return err
 		}
-		src, err := image.FromSource(rawSource)
+		src, err := image.FromSource(ctx, rawSource)
 		if err != nil {
 			if closeErr := rawSource.Close(); closeErr != nil {
 				return errors.Wrapf(err, " (close error: %v)", closeErr)

cmd/skopeo/main.go

@@ -50,6 +50,16 @@ func createApp() *cli.App {
 			Value: "",
 			Usage: "use registry configuration files in `DIR` (e.g. for container signature storage)",
 		},
+		cli.StringFlag{
+			Name:  "override-arch",
+			Value: "",
+			Usage: "use `ARCH` instead of the architecture of the machine for choosing images",
+		},
+		cli.StringFlag{
+			Name:  "override-os",
+			Value: "",
+			Usage: "use `OS` instead of the running OS for choosing images",
+		},
 	}
 	app.Before = func(c *cli.Context) error {
 		if c.GlobalBool("debug") {

cmd/skopeo/utils.go

@@ -12,6 +12,8 @@ import (
 func contextFromGlobalOptions(c *cli.Context, flagPrefix string) (*types.SystemContext, error) {
 	ctx := &types.SystemContext{
 		RegistriesDirPath:  c.GlobalString("registries.d"),
+		ArchitectureChoice: c.GlobalString("override-arch"),
+		OSChoice:           c.GlobalString("override-os"),
 		DockerCertPath:     c.String(flagPrefix + "cert-dir"),
 		// DEPRECATED: keep this here for backward compatibility, but override
 		// them if per subcommand flags are provided (see below).

completions/bash/skopeo

@@ -103,6 +103,8 @@ _skopeo_skopeo() {
      local options_with_args="
 	   --policy
 	   --registries.d
+       --override-arch
+       --override-os
      "
      local boolean_options="
 	   --insecure-policy

docs/skopeo.1.md

@@ -43,6 +43,10 @@ Most commands refer to container images, using a _transport_`:`_details_ format.
 
   **--registries.d** _dir_ use registry configuration files in _dir_ (e.g. for container signature storage), overriding the default path.
 
+  **--override-arch** _arch_ Use _arch_ instead of the architecture of the machine for choosing images.
+
+  **--override-os** _OS_ Use _OS_ instead of the running OS for choosing images.
+
   **--help**|**-h** Show help
 
   **--version**|**-v** print the version number