Add docker image

Dockerfile

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+FROM python:3.9-alpine
+RUN mkdir /duplicati_exporter
+RUN apk add --no-cache --virtual .build-deps alpine-sdk linux-headers
+COPY ./src /duplicati_exporter
+RUN cd /duplicati_exporter && pip install --no-cache-dir -r requirements.txt
+RUN apk del .build-deps
+EXPOSE 5000
+ENV FLASK_RUN_HOST=0.0.0.0
+ENV FLASK_ENV=production
+CMD cd /duplicati_exporter && python -m flask run