feat(builder): add docker download checksum

Author: mboersma

builder/rootfs/Dockerfile

@@ -26,8 +26,12 @@ RUN apk add --update-cache \
     && rm -rf /var/cache/apk/*
 
 # the docker package in alpine disables aufs and devicemapper
-RUN curl -sSL https://get.docker.com/builds/Linux/x86_64/docker-1.8.3 -o /usr/bin/docker && \
-  chmod +x /usr/bin/docker
+ENV DOCKER_BUCKET get.docker.com
+ENV DOCKER_VERSION 1.8.3
+ENV DOCKER_SHA256 f024bc65c45a3778cf07213d26016075e8172de8f6e4b5702bedde06c241650f
+RUN curl -sSL "https://${DOCKER_BUCKET}/builds/Linux/x86_64/docker-$DOCKER_VERSION" -o /usr/bin/docker \
+	&& echo "${DOCKER_SHA256} /usr/bin/docker" | sha256sum -c - \
+	&& chmod +x /usr/bin/docker
 
 # configure ssh server
 RUN mkdir -p /var/run/sshd && rm -rf /etc/ssh/ssh_host*