Add Dockerfile for cross compilation

contrib/Dockerfile

@@ -0,0 +1,38 @@
+# Cross compilation environment for librespot
+# Build the docker image from the root of the project with the following command :
+# $ docker build -t librespot-cross -f contrib/Dockerfile .
+#
+# The resulting image can be used to build librespot for linux x86_64, armhf and armel.
+# $ docker run -v /tmp/librespot-build:/build librespot-cross
+#
+# The compiled binaries will be located in /tmp/librespot-build
+#
+# If only one architecture is desired, cargo can be invoked directly with the appropriate options :
+# $ docker run -v /tmp/librespot-build:/build librespot-cross cargo build --release --no-default-features --features "with-syntex alsa-backend"
+# $ docker run -v /tmp/librespot-build:/build librespot-cross cargo build --release --target arm-unknown-linux-gnueabihf --no-default-features --features "with-syntex alsa-backend"
+# $ docker run -v /tmp/librespot-build:/build librespot-cross cargo build --release --target arm-unknown-linux-gnueabi --no-default-features --features "with-syntex alsa-backend"
+#
+
+FROM debian:stretch
+
+RUN dpkg --add-architecture armhf
+RUN dpkg --add-architecture armel
+RUN apt-get update
+
+RUN apt-get install -y curl build-essential crossbuild-essential-armhf crossbuild-essential-armel
+RUN apt-get install -y libasound2-dev libasound2-dev:armhf libasound2-dev:armel
+
+RUN curl https://sh.rustup.rs -sSf | sh -s -- -y
+ENV PATH="/root/.cargo/bin/:${PATH}"
+RUN rustup target add arm-unknown-linux-gnueabi
+RUN rustup target add arm-unknown-linux-gnueabihf
+
+RUN echo '[target.arm-unknown-linux-gnueabihf]\nlinker = "arm-linux-gnueabihf-gcc"' > ~/.cargo/config
+RUN echo '[target.arm-unknown-linux-gnueabi]\nlinker = "arm-linux-gnueabi-gcc"' > ~/.cargo/config
+
+RUN mkdir /build
+ENV CARGO_TARGET_DIR /build
+
+ADD . /src
+WORKDIR /src
+CMD ["/src/contrib/docker-build.sh"]

contrib/docker-build.sh

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+#!/usr/bin/env bash
+set -eux
+
+cargo build --release --no-default-features --features "with-syntex alsa-backend"
+cargo build --release --target arm-unknown-linux-gnueabihf --no-default-features --features "with-syntex alsa-backend"
+cargo build --release --target arm-unknown-linux-gnueabi --no-default-features --features "with-syntex alsa-backend"

contrib/librespot.service

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+[Unit]
+Description=Librespot
+Requires=network-online.target
+After=network-online.target
+
+[Service]
+User=nobody
+Group=audio
+Restart=always
+RestartSec=10
+ExecStart=/usr/bin/librespot -n "%p on %H"
+
+[Install]
+WantedBy=multi-user.target
+