How to simulate a database migration init container mock in Python

```python
class MigrationJob:
    def __init__(self, name, steps):
        self.name = name
        self.steps = steps
        self.current_step = 0
        self.status = "pending"

    def run(self):
        print(f"Initializing migration job: {self.name}")
        for step in self.steps:
            self.current_step += 1
            self.status = "running"
            print(f"  Step {self.current_step}/{len(self.steps)}: {step}")
            self._execute_step(step)
        self.status = "completed"
        print(f"Migration job {self.name} completed successfully.")

    def _execute_step(self, step):
        required_actions = {
            "schema": "Creating schema",
            "data": "Migrating data",
            "index": "Building indexes",
            "seed": "Seeding initial data",
            "validate": "Validating integrity"
        }
        action = required_actions.get(step, "Executing")
        print(f"    -> {action}...")

    def rollback(self):
        self.status = "rolled_back"
        print(f"Migration job {self.name} rolled back.")


def init_container():
    """Mock init container that runs migration checks before main app starts."""
    print("--- Init Container Starting ---")
    env_checks = ["DB_HOST", "DB_PORT", "DB_NAME", "DB_USER"]
    
    for var in env_checks:
        print(f"Checking environment variable: {var}")
    
    migration = MigrationJob("db_v1_to_v2", ["schema", "data", "index", "seed", "validate"])
    migration.run()
    
    if migration.status == "completed":
        print("--- Init Container Finished: Ready for main container ---")
    else:
        print("--- Init Container Failed: Aborting start ---")
        raise SystemExit(1)


if __name__ == "__main__":
    init_container()