docker ps: allow formatting as JSON

With this patch:

    docker ps --format 'table {{.Names}}\t{{.Platform}}'
    NAMES                    PLATFORM
    optimistic_nightingale   linux/arm64
    mycontainer              linux/arm64/v8
    trusting_goldstine       linux/arm64

    docker ps --format '{{.Platform}}'
    linux/arm64
    linux/arm64/v8
    linux/arm64

    docker ps --format '{{json .Platform}}'
    {"architecture":"arm64","os":"linux"}
    {"architecture":"arm64","os":"linux","variant":"v8"}
    {"architecture":"arm64","os":"linux"}

    docker ps --format 'json'
    {"Command":"\"/bin/bash\"","CreatedAt":"2025-05-13 10:12:19 +0000 UTC","ID":"e8b3b2d604f1","Image":"docker-cli-dev","Labels":"desktop.docker.io/binds/0/Source=/Users/thajeztah/go/src/github.com/docker/cli,desktop.docker.io/binds/0/SourceKind=hostFile,desktop.docker.io/binds/0/Target=/go/src/github.com/docker/cli,desktop.docker.io/mounts/0/Source=/var/run/docker.sock,desktop.docker.io/mounts/0/SourceKind=dockerSocketProxied,desktop.docker.io/mounts/0/Target=/var/run/docker.sock,desktop.docker.io/ports.scheme=v2","LocalVolumes":"1","Mounts":"/host_mnt/User…,docker-cli-dev…,/run/host-serv…","Names":"optimistic_nightingale","Networks":"bridge","Platform":{"architecture":"arm64","os":"linux"},"Ports":"","RunningFor":"38 minutes ago","Size":"0B","State":"running","Status":"Up 38 minutes"}
    {"Command":"\"/docker-entrypoint.…\"","CreatedAt":"2025-05-13 09:58:01 +0000 UTC","ID":"c93b808dd54e","Image":"nginx:alpine","Labels":"desktop.docker.io/ports.scheme=v2,maintainer=NGINX Docker Maintainers \[email protected]\u003e","LocalVolumes":"0","Mounts":"","Names":"mycontainer","Networks":"bridge","Platform":{"architecture":"arm64","os":"linux","variant":"v8"},"Ports":"80/tcp","RunningFor":"53 minutes ago","Size":"0B","State":"running","Status":"Up 53 minutes"}
    {"Command":"\"/usr/bin/gotty --ti…\"","CreatedAt":"2025-05-13 07:31:18 +0000 UTC","ID":"cbb981b06e46","Image":"thajeztah/dockershell:latest","Labels":"desktop.docker.io/ports.scheme=v2,com.thajeztah.docker-shell=1","LocalVolumes":"0","Mounts":"","Names":"trusting_goldstine","Networks":"bridge","Platform":{"architecture":"arm64","os":"linux"},"Ports":"0.0.0.0:55000-\u003e8080/tcp","RunningFor":"3 hours ago","Size":"0B","State":"running","Status":"Up 3 hours"}

Signed-off-by: Sebastiaan van Stijn <[email protected]>

Author: thaJeztah

cli/command/formatter/container.go

@@ -16,6 +16,7 @@ import (
 	"github.com/docker/docker/api/types/container"
 	"github.com/docker/docker/pkg/stringid"
 	"github.com/docker/go-units"
+	v1 "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
 const (
@@ -213,13 +214,24 @@ func (c *ContainerContext) RunningFor() string {
 
 // Platform returns a human-readable representation of the platform
 // of the container if it is available.
-//
-// TODO(thaJeztah): should this take arguments to control "how" it's formatted?
-func (c *ContainerContext) Platform() string {
+func (c *ContainerContext) Platform() Platform {
 	if c.c.ImageManifestDescriptor != nil && c.c.ImageManifestDescriptor.Platform != nil {
-		return platforms.FormatAll(*c.c.ImageManifestDescriptor.Platform)
+		return Platform{*c.c.ImageManifestDescriptor.Platform}
+	}
+	return Platform{}
+}
+
+// Platform wraps a [v1.Platform] to implement the stringer interface.
+type Platform struct {
+	v1.Platform
+}
+
+func (p Platform) String() string {
+	if p.Platform.OS == "" {
+		// Prevent rendering as "unknown"
+		return ""
 	}
-	return ""
+	return platforms.FormatAll(p.Platform)
 }
 
 // Ports returns a comma-separated string representing open ports of the container

cli/command/formatter/container_test.go

@@ -14,6 +14,7 @@ import (
 	"github.com/docker/cli/internal/test"
 	"github.com/docker/docker/api/types/container"
 	"github.com/docker/docker/pkg/stringid"
+	ocispec "github.com/opencontainers/image-spec/specs-go/v1"
 	"gotest.tools/v3/assert"
 	is "gotest.tools/v3/assert/cmp"
 	"gotest.tools/v3/golden"
@@ -420,7 +421,7 @@ func TestContainerContextWriteJSON(t *testing.T) {
 	unix := time.Now().Add(-65 * time.Second).Unix()
 	containers := []container.Summary{
 		{ID: "containerID1", Names: []string{"/foobar_baz"}, Image: "ubuntu", Created: unix, State: "running"},
-		{ID: "containerID2", Names: []string{"/foobar_bar"}, Image: "ubuntu", Created: unix, State: "running"},
+		{ID: "containerID2", Names: []string{"/foobar_bar"}, Image: "ubuntu", Created: unix, State: "running", ImageManifestDescriptor: &ocispec.Descriptor{Platform: &ocispec.Platform{Architecture: "amd64", OS: "linux"}}},
 	}
 	expectedCreated := time.Unix(unix, 0).String()
 	expectedJSONs := []map[string]any{
@@ -434,7 +435,7 @@ func TestContainerContextWriteJSON(t *testing.T) {
 			"Mounts":       "",
 			"Names":        "foobar_baz",
 			"Networks":     "",
-			"Platform":     "",
+			"Platform":     map[string]any{"architecture": "", "os": ""},
 			"Ports":        "",
 			"RunningFor":   "About a minute ago",
 			"Size":         "0B",
@@ -451,7 +452,7 @@ func TestContainerContextWriteJSON(t *testing.T) {
 			"Mounts":       "",
 			"Names":        "foobar_bar",
 			"Networks":     "",
-			"Platform":     "",
+			"Platform":     map[string]any{"architecture": "amd64", "os": "linux"},
 			"Ports":        "",
 			"RunningFor":   "About a minute ago",
 			"Size":         "0B",