Dockerhub rate limiting impacting libffi testing -- solutions?

[Anthony Green]

libffi CI testing relies heavily on OCI-compatible images containing cross
toolchains and emulators.  Those container images are hosted in
dockerhub and pulled down for use on travis-ci.   It only takes a few PRs
or commits for us to exceed our free dockerhub daily quota.   I don't have
a lot of time to look into this, so I'm hoping somebody out there has an
informed opinion on where we can rehost those images for free, or solve the
rate-limiting problem with dockerhub.

I want to solve this quickly in order to get a new release out before too
long.

Thank you for your ideas!

AG