Pin the docker version for multiarch builds

It seems that letting CircleCI use its default docker version (17.09.0-ce,
apparently) did not interact well with multiarch builds: in particular, we saw
weird effects where running an amd64 build at the same time as an arm64 build
caused the arm64 builds to fail with:

   Error while loading /usr/sbin/dpkg-deb: No such file or directory