The docker package installed on our contint* hosts is terribly out of date relative to the rest of the CI infrastructure and needs a newer package installed from thirdparty/ci.
The migration of contint1001 and contint2001 to buster effectively downgraded its docker package, since we'd been using thirdparty/ci for stretch (version 19.03) but started using the distro package with buster (version 18.09). Our runners are now running bullseye and docker 20.10. Since contint servers are the only runners allowed to publish images, their docker packages need to be kept as up-to-date as possible with other runners.