Add dockerfile for cross compiling arm32v7 image

contrib/linuxarm32v7.Dockerfile

@@ -0,0 +1,108 @@
+# This dockerfile is meant to cross compile with a x64 machine for a arm32v7 host
+# It is using multi stage build: 
+# * downloader: Download litecoin/bitcoin and qemu binaries needed for c-lightning
+# * builder: Cross compile c-lightning dependencies, then c-lightning itself with static linking
+# * final: Copy the binaries required at runtime
+# The resulting image uploaded to dockerhub will only contain what is needed for runtime.
+# From the root of the repository, run "docker build -t yourimage:yourtag -f contrib/linuxarm32v7.Dockerfile ."
+FROM debian:stretch-slim as downloader
+
+RUN set -ex \
+	&& apt-get update \
+	&& apt-get install -qq --no-install-recommends ca-certificates dirmngr wget \
+     qemu qemu-user-static qemu-user binfmt-support
+
+WORKDIR /opt
+
+ARG BITCOIN_VERSION=0.17.0
+ENV BITCOIN_TARBALL bitcoin-$BITCOIN_VERSION-arm-linux-gnueabihf.tar.gz
+ENV BITCOIN_URL https://bitcoincore.org/bin/bitcoin-core-$BITCOIN_VERSION/$BITCOIN_TARBALL
+ENV BITCOIN_ASC_URL https://bitcoincore.org/bin/bitcoin-core-$BITCOIN_VERSION/SHA256SUMS.asc
+
+RUN mkdir /opt/bitcoin && cd /opt/bitcoin \
+    && wget -qO $BITCOIN_TARBALL "$BITCOIN_URL" \
+    && wget -qO bitcoin.asc "$BITCOIN_ASC_URL" \
+    && grep $BITCOIN_TARBALL bitcoin.asc | tee SHA256SUMS.asc \
+    && sha256sum -c SHA256SUMS.asc \
+    && BD=bitcoin-$BITCOIN_VERSION/bin \
+    && tar -xzvf $BITCOIN_TARBALL $BD/bitcoin-cli --strip-components=1 \
+    && rm $BITCOIN_TARBALL
+
+ENV LITECOIN_VERSION 0.14.2
+ENV LITECOIN_TARBALL litecoin-$LITECOIN_VERSION-arm-linux-gnueabihf.tar.gz
+ENV LITECOIN_URL https://download.litecoin.org/litecoin-$LITECOIN_VERSION/linux/$LITECOIN_TARBALL
+ENV LITECOIN_SHA256 e79f2a8e8e1b9920d07cff8482237b56aa4be2623103d3d2825ce09a2cc2f6d7
+
+# install litecoin binaries
+RUN mkdir /opt/litecoin && cd /opt/litecoin \
+    && wget -qO litecoin.tar.gz "$LITECOIN_URL" \
+    && echo "$LITECOIN_SHA256  litecoin.tar.gz" | sha256sum -c - \
+    && BD=litecoin-$LITECOIN_VERSION/bin \
+    && tar -xzvf litecoin.tar.gz $BD/litecoin-cli --strip-components=1 --exclude=*-qt \
+    && rm litecoin.tar.gz
+
+FROM debian:stretch-slim as builder
+
+ENV LIGHTNINGD_VERSION=master
+RUN apt-get update && apt-get install -y --no-install-recommends ca-certificates autoconf automake build-essential git libtool python python3 wget gnupg dirmngr git \
+  libc6-armhf-cross gcc-arm-linux-gnueabihf g++-arm-linux-gnueabihf
+
+ENV target_host=arm-linux-gnueabihf
+
+ENV AR=${target_host}-ar \
+AS=${target_host}-as \
+CC=${target_host}-gcc \
+CXX=${target_host}-g++ \
+LD=${target_host}-ld \
+STRIP=${target_host}-strip \
+QEMU_LD_PREFIX=/usr/${target_host} \
+HOST=${target_host}
+
+RUN wget -q https://zlib.net/zlib-1.2.11.tar.gz \
+&& tar xvf zlib-1.2.11.tar.gz \
+&& cd zlib-1.2.11 \
+&& ./configure --prefix=$QEMU_LD_PREFIX \
+&& make \
+&& make install && cd .. && rm zlib-1.2.11.tar.gz && rm -rf zlib-1.2.11
+
+RUN apt-get install -y --no-install-recommends unzip tclsh \
+&& wget -q https://www.sqlite.org/2018/sqlite-src-3260000.zip \
+&& unzip sqlite-src-3260000.zip \
+&& cd sqlite-src-3260000 \
+&& ./configure --enable-static --disable-readline --disable-threadsafe --disable-load-extension --host=${target_host} --prefix=$QEMU_LD_PREFIX \
+&& make \
+&& make install && cd .. && rm sqlite-src-3260000.zip && rm -rf sqlite-src-3260000
+
+RUN wget -q https://gmplib.org/download/gmp/gmp-6.1.2.tar.xz \
+&& tar xvf gmp-6.1.2.tar.xz \
+&& cd gmp-6.1.2 \
+&& ./configure --disable-assembly --prefix=$QEMU_LD_PREFIX --host=${target_host} \
+&& make \
+&& make install && cd .. && rm gmp-6.1.2.tar.xz && rm -rf gmp-6.1.2
+
+COPY --from=downloader /usr/bin/qemu-arm-static /usr/bin/qemu-arm-static
+WORKDIR /opt/lightningd
+COPY . .
+ARG DEVELOPER=0
+RUN ./configure --enable-static && make -j3 DEVELOPER=${DEVELOPER} && cp lightningd/lightning* cli/lightning-cli /usr/bin/
+
+FROM arm32v7/debian:stretch-slim as final
+COPY --from=downloader /usr/bin/qemu-arm-static /usr/bin/qemu-arm-static
+RUN apt-get update && apt-get install -y --no-install-recommends socat inotify-tools \
+    && rm -rf /var/lib/apt/lists/* 
+
+ENV LIGHTNINGD_DATA=/root/.lightning
+ENV LIGHTNINGD_PORT=9835
+
+RUN mkdir $LIGHTNINGD_DATA && \
+    touch $LIGHTNINGD_DATA/config
+VOLUME [ "/root/.lightning" ]
+
+COPY --from=builder /opt/lightningd/cli/lightning-cli /usr/bin
+COPY --from=builder /opt/lightningd/lightningd/lightning* /usr/bin/
+COPY --from=downloader /opt/bitcoin/bin /usr/bin
+COPY --from=downloader /opt/litecoin/bin /usr/bin
+COPY tools/docker-entrypoint.sh entrypoint.sh
+
+EXPOSE 9735 9835
+ENTRYPOINT  [ "./entrypoint.sh" ]