Initial commit: add Modbus slave simulator

- TCP/RTU/TLS server support with pymodbus
- Configurable data store sizes and initial values
- CLI with argument parsing for port, slave-id, etc.
- GitHub Actions CI/CD for Docker build and push

pyproject.toml

@@ -0,0 +1,7 @@
+[project]
+name = "modbus-slave"
+version = "0.1.0"
+requires-python = ">=3.9"
+dependencies = [
+    "pymodbus>=3.8.6",
+]