feat: implement Stringer on custom string types

Add String() methods to ImageID, ContainerID, and UnparsedURL.
Replace all string() casts with .String() method calls throughout
the codebase for cleaner interface compliance.

internal/docker/types.go

@@ -3,5 +3,11 @@ package docker
 // ImageID is a Docker image identifier (ID or tag).
 type ImageID string
 
+// String implements fmt.Stringer.
+func (id ImageID) String() string { return string(id) }
+
 // ContainerID is a Docker container identifier.
 type ContainerID string
+
+// String implements fmt.Stringer.
+func (id ContainerID) String() string { return string(id) }