Move interop test Docker files to tests/docker/ #855
Description
Summary
Follow-up from #839. Move the Docker Compose files (docker-compose.yml, docker-compose-lnd.yml, docker-compose-cln.yml) into a dedicated tests/docker/ directory for better organization.
Context
Currently the Docker Compose files live at the repo root. Moving them under tests/docker/ keeps the root clean and groups all test infrastructure together.
Ref: #839 (comment)