Adds a Dockerfile for building a lightweight Docker container based on alpine

Dockerfile

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+FROM alpine:3.5
+WORKDIR /app/build
+COPY . /app
+RUN apk add --no-cache g++ make cmake libmpdclient-dev openssl-dev
+RUN cmake ..
+RUN make
+
+FROM alpine:3.5
+RUN apk add  --no-cache libmpdclient openssl
+EXPOSE 8080
+COPY --from=0 /app/build/ympd /usr/bin/ympd
+COPY --from=0 /app/build/mkdata /usr/bin/mkdata
+CMD ympd