Add Dockerfile.Rpi which creates a librespot docker image which can be used on the RPI directly.

contrib/Dockerfile.Rpi

@@ -0,0 +1,54 @@
+# Create a docker image for the RPI
+# Build the docker image from the root of the project with the following command :
+# $ docker build -t librespot-rpi -f .\contrib\Dockerfile.Rpi .
+#
+# This builds a docker image which is usable when running docker on the rpi.
+# 
+# This Dockerfile builds in windows without any requirements, for linux based systems you might need to run the following line:
+# docker run --rm --privileged multiarch/qemu-user-static:register --reset
+# (see here for more info: https://gist.github.com/PieterScheffers/d50f609d9628383e4c9d8d7d269b7643 )
+#
+# Save the docker image to a file:
+# $ docker save -o contrib/librespot-rpi librespot-rpi
+#
+# Move it to the rpi and import it with:
+# docker load -i librespot-rpi
+#
+# Run it with:
+# docker run -d --restart unless-stopped $(for DEV in $(find /dev/snd -type c); do echo --device=$DEV:$DEV; done) --net=host --name librespot-rpi librespot-rpi --name {devicename} 
+
+FROM debian:stretch
+
+RUN dpkg --add-architecture armhf
+RUN apt-get update
+
+RUN apt-get install -y curl git build-essential crossbuild-essential-armhf
+RUN apt-get install -y libasound2-dev libasound2-dev:armhf
+
+RUN curl https://sh.rustup.rs -sSf | sh -s -- -y
+ENV PATH="/root/.cargo/bin/:${PATH}"
+RUN rustup target add arm-unknown-linux-gnueabihf
+
+RUN mkdir /.cargo && \
+    echo '[target.arm-unknown-linux-gnueabihf]\nlinker = "arm-linux-gnueabihf-gcc"' >> /.cargo/config
+
+RUN mkdir /build
+ENV CARGO_TARGET_DIR /build
+ENV CARGO_HOME /build/cache
+
+ADD . /src
+WORKDIR /src
+RUN cargo build --release --target arm-unknown-linux-gnueabihf --no-default-features --features "alsa-backend"
+
+
+FROM resin/rpi-raspbian
+RUN apt-get update && \
+    apt-get install libasound2 && \
+    rm -rf /var/lib/apt/lists/*
+
+RUN mkdir /librespot
+WORKDIR /librespot
+
+COPY --from=0 /build/arm-unknown-linux-gnueabihf/release/librespot .
+RUN chmod +x librespot
+ENTRYPOINT ["./librespot"]