Added Dockerfile and instructions to build it

Dockerfile

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+FROM python:3.13-slim
+RUN useradd --create-home --shell /bin/bash solarcontrol
+WORKDIR /script
+COPY solarcontrol.py ./
+RUN pip install --upgrade pip && pip3 install --no-cache-dir paho-mqtt python-dotenv requests
+USER solarcontrol
+ENTRYPOINT ["/usr/bin/python3", "solarcontrol.py"]