Switch to squashfs for tiny rootfs! (#438)

* try squashfs

* rw

* build squash directly

* oops

* cleanup

Dockerfile.builder

@@ -18,6 +18,7 @@ RUN apt-get update && \
     openssl \
     ccache \
     libcap2-bin \
+    squashfs-tools \
     && rm -rf /var/lib/apt/lists/*
 
 RUN if [ ${UID:-0} -ne 0 ] && [ ${GID:-0} -ne 0 ]; then \