Shrink Docker Image, bump version, docker run as non-root

Dockerfile

@@ -1,4 +1,6 @@
-FROM python:3
+FROM python:3-slim
+
+RUN useradd sungather
 
 WORKDIR /usr/src/sungather
 
@@ -6,6 +8,10 @@ COPY requirements.txt ./
 RUN pip install --no-cache-dir --upgrade -r requirements.txt
 
 COPY SunGather/ .
-RUN touch /config/config.yaml
 
-CMD [ "python", "sungather.py -c /config/config.yaml" ]
+VOLUME /config
+COPY SunGather/config-example.yaml /config/config.yaml
+
+USER sungather
+
+CMD [ "python", "sungather.py", "-c", "/config/config.yaml" ]