script to build arm images

CHANGELOG.md.20190330

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+find . -iname 'dockerfile' -exec sed -i "s/FROM.*/ARG DISTRO=alpine:3.8\nFROM \$DISTRO/" '{}' \;

tests/build_arm.sh

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+#!/bin/bash -x
+
+export DISTRO="hypriot/rpi-alpine"
+docker-compose -f build.yml build --build-arg DISTRO=$DISTRO # --parallel