Googling the error message
it looks like I have an older version of docker.io than the docker image docker is trying to pull.
I've found the public image, but there was no version there that I could see: https://hub.docker.com/r/tudorg/beats-builder/
The version of docker.io that was installed on the rPi, was 1.3.3 :
# dpkg -l docker.io
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
+++-========================-=================-=================-======================================================
ii docker.io 1.3.3~dfsg1-2 armhf Linux container runtime