Add String() methods to ImageID, ContainerID, and UnparsedURL.
Replace all string() casts with .String() method calls throughout
the codebase for cleaner interface compliance.
Move ImageID and ContainerID into internal/docker/types.go (where they're
primarily used) and UnparsedURL into internal/service/webhook/types.go.
This follows the Go convention of defining types alongside the code that
uses them rather than in a separate 'domain' package.