Refactor project structure and enhance Docker configuration

- Updated `.dockerignore` to exclude test and development artifacts, optimizing the Docker image size.
- Refactored `main.py` to delegate execution to `duty_teller.run.main()`, simplifying the entry point.
- Introduced a new `duty_teller` package to encapsulate core functionality, improving modularity and organization.
- Enhanced `pyproject.toml` to define a script for running the application, streamlining the execution process.
- Updated README documentation to reflect changes in project structure and usage instructions.
- Improved Alembic environment configuration to utilize the new package structure for database migrations.

tests/test_duty_schedule_parser.py

@@ -4,7 +4,7 @@ from datetime import date
 
 import pytest
 
-from importers.duty_schedule import (
+from duty_teller.importers.duty_schedule import (
     DUTY_MARKERS,
     UNAVAILABLE_MARKER,
     VACATION_MARKER,