image/save: Add `--platform`

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/image/save.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"io"
 
+	"github.com/containerd/platforms"
 	"github.com/docker/cli/cli"
 	"github.com/docker/cli/cli/command"
 	"github.com/docker/cli/cli/command/completion"
@@ -13,8 +14,9 @@ import (
 )
 
 type saveOptions struct {
-	images []string
+	images   []string
-	output string
+	output   string
+	platform string
 }
 
 // NewSaveCommand creates a new `docker save` command
@@ -38,7 +40,10 @@ func NewSaveCommand(dockerCli command.Cli) *cobra.Command {
 	flags := cmd.Flags()
 
 	flags.StringVarP(&opts.output, "output", "o", "", "Write to a file, instead of STDOUT")
+	flags.StringVar(&opts.platform, "platform", "", `Save only the given platform variant. Formatted as "os[/arch[/variant]]" (e.g., "linux/amd64")`)
+	_ = flags.SetAnnotation("platform", "version", []string{"1.48"})
 
+	_ = cmd.RegisterFlagCompletionFunc("platform", completion.Platforms)
 	return cmd
 }
 
@@ -52,7 +57,16 @@ func RunSave(ctx context.Context, dockerCli command.Cli, opts saveOptions) error
 		return errors.Wrap(err, "failed to save image")
 	}
 
-	responseBody, err := dockerCli.Client().ImageSave(ctx, opts.images, image.SaveOptions{})
+	var options image.SaveOptions
+	if opts.platform != "" {
+		p, err := platforms.Parse(opts.platform)
+		if err != nil {
+			return errors.Wrap(err, "invalid platform")
+		}
+		options.Platform = &p
+	}
+
+	responseBody, err := dockerCli.Client().ImageSave(ctx, opts.images, options)
 	if err != nil {
 		return err
 	}

cli/command/image/save_test.go

@@ -8,6 +8,7 @@ import (
 
 	"github.com/docker/cli/internal/test"
 	"github.com/docker/docker/api/types/image"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 	"github.com/pkg/errors"
 	"gotest.tools/v3/assert"
 	is "gotest.tools/v3/assert/cmp"
@@ -51,6 +52,11 @@ func TestNewSaveCommandErrors(t *testing.T) {
 			args:          []string{"-o", "/dev/null", "arg1"},
 			expectedError: "failed to save image: invalid output path: \"/dev/null\" must be a directory or a regular file",
 		},
+		{
+			name:          "invalid platform",
+			args:          []string{"--platform", "<invalid>", "arg1"},
+			expectedError: `invalid platform`,
+		},
 	}
 	for _, tc := range testCases {
 		tc := tc
@@ -95,6 +101,16 @@ func TestNewSaveCommandSuccess(t *testing.T) {
 				return io.NopCloser(strings.NewReader("")), nil
 			},
 		},
+		{
+			args:       []string{"--platform", "linux/amd64", "arg1"},
+			isTerminal: false,
+			imageSaveFunc: func(images []string, options image.SaveOptions) (io.ReadCloser, error) {
+				assert.Assert(t, is.Len(images, 1))
+				assert.Check(t, is.Equal("arg1", images[0]))
+				assert.Check(t, is.DeepEqual(ocispec.Platform{OS: "linux", Architecture: "amd64"}, *options.Platform))
+				return io.NopCloser(strings.NewReader("")), nil
+			},
+		},
 	}
 	for _, tc := range testCases {
 		tc := tc

docs/reference/commandline/image_save.md

@@ -9,9 +9,10 @@ Save one or more images to a tar archive (streamed to STDOUT by default)
 
 ### Options
 
-| Name             | Type     | Default | Description                        |
+| Name                      | Type     | Default | Description                                                                                    |
-|:-----------------|:---------|:--------|:-----------------------------------|
+|:--------------------------|:---------|:--------|:-----------------------------------------------------------------------------------------------|
-| `-o`, `--output` | `string` |         | Write to a file, instead of STDOUT |
+| `-o`, `--output`          | `string` |         | Write to a file, instead of STDOUT                                                             |
+| [`--platform`](#platform) | `string` |         | Save only the given platform variant. Formatted as `os[/arch[/variant]]` (e.g., `linux/amd64`) |
 
 
 <!---MARKER_GEN_END-->
@@ -59,3 +60,52 @@ You can even cherry-pick particular tags of an image repository.
 ```console
 $ docker save -o ubuntu.tar ubuntu:lucid ubuntu:saucy
 ```
+
+### <a name="platform"></a> Save a specific platform (--platform)
+
+The `--platform` option allows you to specify which platform variant of the
+image to save. By default, `docker save` saves all platform variants that
+are present in the daemon's image store. Use the `--platform` option
+to specify which platform variant of the image to save. An error is produced
+if the given platform is not present in the local image store.
+
+The platform option takes the `os[/arch[/variant]]` format; for example,
+`linux/amd64` or `linux/arm64/v8`. Architecture and variant are optional,
+and default to the daemon's native architecture if omitted.
+
+The following example pulls the RISC-V variant of the `alpine:latest` image
+and saves it to a tar archive.
+
+```console
+$ docker pull --platform=linux/riscv64 alpine:latest
+latest: Pulling from library/alpine
+8c4a05189a5f: Download complete 
+Digest: sha256:beefdbd8a1da6d2915566fde36db9db0b524eb737fc57cd1367effd16dc0d06d
+Status: Downloaded newer image for alpine:latest
+docker.io/library/alpine:latest
+
+$ docker image save --platform=linux/riscv64 -o alpine-riscv.tar alpine:latest
+
+$ ls -lh image.tar
+-rw-------  1 thajeztah  staff   3.9M Oct  7 11:06 alpine-riscv.tar
+```
+
+The following example attempts to save a platform variant of `alpine:latest`
+that doesn't exist in the local image store, resulting in an error.
+
+```console
+$ docker image ls --tree
+IMAGE                   ID             DISK USAGE   CONTENT SIZE   IN USE
+alpine:latest           beefdbd8a1da       10.6MB         3.37MB
+├─ linux/riscv64        80cde017a105       10.6MB         3.37MB
+├─ linux/amd64          33735bd63cf8           0B             0B
+├─ linux/arm/v6         50f635c8b04d           0B             0B
+├─ linux/arm/v7         f2f82d424957           0B             0B
+├─ linux/arm64/v8       9cee2b382fe2           0B             0B
+├─ linux/386            b3e87f642f5c           0B             0B
+├─ linux/ppc64le        c7a6800e3dc5           0B             0B
+└─ linux/s390x          2b5b26e09ca2           0B             0B
+
+$ docker image save --platform=linux/s390x -o alpine-s390x.tar alpine:latest
+Error response from daemon: no suitable export target found for platform linux/s390x
+```

docs/reference/commandline/save.md

@@ -9,9 +9,10 @@ Save one or more images to a tar archive (streamed to STDOUT by default)
 
 ### Options
 
-| Name             | Type     | Default | Description                        |
+| Name             | Type     | Default | Description                                                                                    |
-|:-----------------|:---------|:--------|:-----------------------------------|
+|:-----------------|:---------|:--------|:-----------------------------------------------------------------------------------------------|
-| `-o`, `--output` | `string` |         | Write to a file, instead of STDOUT |
+| `-o`, `--output` | `string` |         | Write to a file, instead of STDOUT                                                             |
+| `--platform`     | `string` |         | Save only the given platform variant. Formatted as `os[/arch[/variant]]` (e.g., `linux/amd64`) |
 
 
 <!---MARKER_GEN_END-->