Add Docker specific tweaks

Dockerfile

 FROM scratch
 ADD rootfs.tar.xz /
+
 COPY qemu-arm-static /usr/bin/qemu-arm-static
+
+# a few minor docker-specific tweaks
+# see https://github.com/docker/docker/blob/master/contrib/mkimage/debootstrap
+RUN echo '#!/bin/sh' > /usr/sbin/policy-rc.d \
+ && echo 'exit 101' >> /usr/sbin/policy-rc.d \
+ && chmod +x /usr/sbin/policy-rc.d \
+ && dpkg-divert --local --rename --add /sbin/initctl \
+ && cp -a /usr/sbin/policy-rc.d /sbin/initctl \
+ && sed -i 's/^exit.*/exit 0/' /sbin/initctl \
+ && echo 'force-unsafe-io' > /etc/dpkg/dpkg.cfg.d/docker-apt-speedup \
+ && echo 'DPkg::Post-Invoke { "rm -f /var/cache/apt/archives/*.deb /var/cache/apt/archives/partial/*.deb /var/cache/apt/*.bin || true"; };' > /etc/apt/apt.conf.d/docker-clean \
+ && echo 'APT::Update::Post-Invoke { "rm -f /var/cache/apt/archives/*.deb /var/cache/apt/archives/partial/*.deb /var/cache/apt/*.bin || true"; };' >> /etc/apt/apt.conf.d/docker-clean \
+ && echo 'Dir::Cache::pkgcache ""; Dir::Cache::srcpkgcache "";' >> /etc/apt/apt.conf.d/docker-clean \
+ && echo 'Acquire::Languages "none";' > /etc/apt/apt.conf.d/docker-no-languages \
+ && echo 'Acquire::GzipIndexes "true"; Acquire::CompressionTypes::Order:: "gz";' > /etc/apt/apt.conf.d/docker-gzip-indexes
+
 CMD ["bash"]