Add docker setup

Dockerfile

@@ -0,0 +1,12 @@
+FROM python:3.9-slim
+
+WORKDIR /app
+
+ENTRYPOINT ["python3", "-m", "cloudflare-ddns"]
+CMD []
+
+# Install requirements in a separate step to not rebuild everything when the requirements are updated.
+COPY requirements.txt ./
+RUN pip install -r requirements.txt
+
+COPY . .