Add docker image

Dockerfile

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+FROM python:3.6-alpine
+
+RUN apk add --no-cache \
+    ffmpeg
+
+ADD requirements.txt /app/
+RUN cd /app && pip install -U -r requirements.txt
+
+ADD spotdl.py /app/
+ADD core/ /app/core
+
+WORKDIR /app
+
+ENTRYPOINT ["python3", "spotdl.py", "-f", "."]