pub fn resolve_levels(file: &ComposeFile) -> Result<Vec<Vec<String>>>Expand description
Group services into dependency levels (Kahn’s algorithm, layered).
Each returned level contains services whose dependencies all live in earlier
levels, so the services within one level have no depends_on relationship to
each other and can be started concurrently. Levels are ordered
dependencies-first; names within a level are sorted for deterministic
dispatch. Errors on cycles or missing required dependencies, matching
resolve_order.