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

Dockerfile

@@ -0,0 +1,18 @@
+FROM python:3.11-slim
+
+WORKDIR /app
+
+# Install dependencies
+COPY pyproject.toml uv.lock ./
+RUN pip install uv && uv sync --frozen
+
+# Copy application
+COPY main.py ./
+
+# Create non-root user
+RUN useradd -m modbus && chown -R modbus:modbus /app
+USER modbus
+
+EXPOSE 5020
+
+CMD ["python", "main.py"]