Resolves the container engine host from a .env file in the project root.
Reads DOCKER_HOST from .env (if present) so that developers can
commit a project-local default without modifying their shell profile.
Only activates when DOCKER_HOST is not 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.