Resolves the container engine host from a .env file in the project root.
Reads CONTAINER_ENGINE_HOST from .env (if present) so that developers can
commit a project-local default without modifying their shell profile.
Falls back to DOCKER_HOST for backward compatibility.
Only activates when neither CONTAINER_ENGINE_HOST nor DOCKER_HOST is
already set in the environment, making this a fallback rather than an override.
The .env file uses simple KEY=VALUE syntax, one per line.
Lines starting with # are treated as comments.