work around 'docker rm -f' weirdness
I've been seeing

~# docker rm -f weave
Error response from daemon: Cannot destroy container weave: Driver devicemapper failed to remove root filesystem 29e46c3e400ee3c8654f2b06a0f6d97bde98dfeb9bb61d25195dba38c04324fc: Device is Busy

whereas the corresponding two-step 'docker kill' + 'docker rm' works
fine.