refactor: eliminate internal/domain package, colocate types with implementations

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.

internal/docker/types.go

@@ -0,0 +1,7 @@
+package docker
+
+// ImageID is a Docker image identifier (ID or tag).
+type ImageID string
+
+// ContainerID is a Docker container identifier.
+type ContainerID string