added libffi-dev as dependency

Dockerfile

 FROM python:3.7-alpine
 
-RUN apk add --no-cache --virtual .build-deps gcc musl-dev
+RUN apk add --no-cache --virtual .build-deps gcc musl-dev libffi-dev
 
 RUN pip install pipenv
 COPY . /opt/SpotifyBackup
@@ -9,6 +9,6 @@ WORKDIR /opt/SpotifyBackup
 RUN pipenv install
 RUN ln -s $(pipenv --venv) /opt/SpotifyBackup/mypipenv
 
-RUN apk del .build-deps gcc musl-dev
+RUN apk del .build-deps gcc musl-dev libffi-dev
 
 CMD [ "/opt/SpotifyBackup/mypipenv/bin/python", "/opt/SpotifyBackup/SpotifyBackup.py" ]
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