Add docker whoami command

[zampani-docker]

Summary

Implements a new docker whoami command that displays the username of the currently logged-in user for Docker registries.

Features

• Display username for Docker Hub by default (when no registry is specified)
• Support specifying a custom registry as an argument: docker whoami [SERVER]
• Add --all flag to list all authenticated registries with usernames
• Proper error handling for non-authenticated registries
• Follows existing patterns from docker login and docker logout commands

Implementation Details

• Added cli/command/registry/whoami.go with main implementation
• Added cli/command/registry/whoami_test.go with comprehensive test coverage (13 test cases)
• All existing tests pass (2365 tests)
• Command appears in help output with appropriate category ordering

Usage Examples

# Show username for Docker Hub
docker whoami

# Show username for a specific registry
docker whoami custom.registry.com

# List all authenticated registries with usernames
docker whoami --all

# Show help
docker whoami --help

Test Coverage

• Not logged in scenarios (Docker Hub and custom registries)
• Logged in scenarios (Docker Hub and custom registries)
• --all flag functionality
• Registry address normalization (with/without protocol, with port)
• Docker.io alias handling
• Empty username handling
• DOCKER_AUTH_CONFIG environment variable warning

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 61.70213% with 18 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cli/command/registry/whoami.go	61.70%	16 Missing and 2 partials ⚠️

📢 Thoughts on this report? Let us know!

[thaJeztah]

Thanks for contributing; I'm not sure if we should add this though, currently; there's work in progress to rewrite all authentication, and as part of that the way credentials are stored (and used) may be updated in the (near) future.

@Benehiko may be able to fill me in on some of that, but it's probably better to keep the status quo for now until that work is completed.