Add Dockerfile

I wrote one already, it works for me, maybe it will help others 😙

```
FROM python:3.8-alpine

WORKDIR /uploads
VOLUME /uploads

RUN apk add gcc musl-dev libffi-dev openssl-dev

RUN pip install updog

EXPOSE 9090

CMD ["updog"]
```

I think it should be better if instead of pip installing it installed the requirements.txt, but I wrote it this way and it works 😅
