cli/command/registry: add whoami command

Add a new 'docker whoami' command that displays the username of the
currently logged-in user for Docker registries. The command follows
existing patterns from login/logout commands.

Features:
- Display username for Docker Hub by default
- Support specifying a custom registry as an argument
- Add --all flag to list all authenticated registries with usernames
- Proper error handling for non-authenticated registries
- Comprehensive test coverage with 13 test cases

Co-Authored-By: Claude Sonnet 4.5 <[email protected]>

Author: zampani-docker

cli/command/registry/whoami.go

@@ -0,0 +1,104 @@
+package registry
+
+import (
+	"context"
+	"errors"
+	"fmt"
+
+	"github.com/docker/cli/cli"
+	"github.com/docker/cli/cli/command"
+	"github.com/docker/cli/cli/config/credentials"
+	"github.com/docker/cli/internal/commands"
+	"github.com/docker/cli/internal/registry"
+	"github.com/spf13/cobra"
+)
+
+func init() {
+	commands.Register(newWhoamiCommand)
+}
+
+type whoamiOptions struct {
+	serverAddress string
+	all           bool
+}
+
+// newWhoamiCommand creates a new `docker whoami` command
+func newWhoamiCommand(dockerCLI command.Cli) *cobra.Command {
+	var opts whoamiOptions
+
+	cmd := &cobra.Command{
+		Use:   "whoami [SERVER]",
+		Short: "Display the username of the currently logged in user",
+		Long:  "Display the username of the currently logged in user.\nDefaults to Docker Hub if no server is specified.",
+		Args:  cli.RequiresMaxArgs(1),
+		RunE: func(cmd *cobra.Command, args []string) error {
+			if len(args) > 0 {
+				opts.serverAddress = args[0]
+			}
+			return runWhoami(cmd.Context(), dockerCLI, opts)
+		},
+		Annotations: map[string]string{
+			"category-top": "10",
+		},
+		ValidArgsFunction:     cobra.NoFileCompletions,
+		DisableFlagsInUseLine: true,
+	}
+
+	flags := cmd.Flags()
+	flags.BoolVar(&opts.all, "all", false, "Display usernames for all authenticated registries")
+
+	return cmd
+}
+
+func runWhoami(_ context.Context, dockerCLI command.Cli, opts whoamiOptions) error {
+	maybePrintEnvAuthWarning(dockerCLI)
+
+	if opts.all {
+		return runWhoamiAll(dockerCLI)
+	}
+	return runWhoamiSingle(dockerCLI, opts)
+}
+
+func runWhoamiSingle(dockerCLI command.Cli, opts whoamiOptions) error {
+	serverAddress := opts.serverAddress
+	if serverAddress == "" || serverAddress == registry.DefaultNamespace {
+		serverAddress = registry.IndexServer
+	} else {
+		serverAddress = credentials.ConvertToHostname(serverAddress)
+	}
+
+	authConfig, err := dockerCLI.ConfigFile().GetAuthConfig(serverAddress)
+	if err != nil {
+		return err
+	}
+
+	if authConfig.Username == "" {
+		registryName := "Docker Hub"
+		if opts.serverAddress != "" && opts.serverAddress != registry.DefaultNamespace {
+			registryName = serverAddress
+		}
+		return fmt.Errorf("not logged in to %s", registryName)
+	}
+
+	fmt.Fprintln(dockerCLI.Out(), authConfig.Username)
+	return nil
+}
+
+func runWhoamiAll(dockerCLI command.Cli) error {
+	creds, err := dockerCLI.ConfigFile().GetAllCredentials()
+	if err != nil {
+		return err
+	}
+
+	if len(creds) == 0 {
+		return errors.New("not logged in to any registries")
+	}
+
+	for serverAddress, authConfig := range creds {
+		if authConfig.Username != "" {
+			fmt.Fprintf(dockerCLI.Out(), "%s: %s\n", serverAddress, authConfig.Username)
+		}
+	}
+
+	return nil
+}