Add Docker command family

[maniramezan]

Summary

• Add a typed Docker command family with global flags, Buildx, Compose, container/image, Debug, MCP, Scout, init, and raw argument escape hatches
• Add mock-only Docker tests and DocC documentation
• Wire the Docker trait into Package.swift, CI trait matrices, README, DocC navigation, and agent guidance

Validation

• swift test -Xswiftc -warnings-as-errors
• swift build -c release -Xswiftc -warnings-as-errors
• swift build -c release --traits Docker -Xswiftc -warnings-as-errors
• swift test --traits Docker -Xswiftc -warnings-as-errors
• swift build -c release --enable-all-traits -Xswiftc -warnings-as-errors
• swift test --enable-all-traits -Xswiftc -warnings-as-errors
• swift test --enable-all-traits --enable-code-coverage -Xswiftc -warnings-as-errors
• swift -warnings-as-errors Scripts/validate-code-coverage.swift --input .build/arm64-apple-macosx/debug/codecov/SwiftyShell.json --minimum-line-coverage 84
• swift -warnings-as-errors Scripts/validate-traits.swift
• swift -warnings-as-errors Scripts/validate-docc-coverage.swift
• swift package -Xswiftc -warnings-as-errors --allow-writing-to-directory docs generate-documentation --target SwiftyShell --output-path docs --transform-for-static-hosting --hosting-base-path SwiftyShell
• swift-format lint --strict Package.swift Sources/SwiftyShell/Docker/Docker.swift Tests/SwiftyShellTests/Docker/DockerTests.swift
• git diff --check