Right, you couldn't run docker/dockerd, but you could run some docker images, probably? They're just filesystems with regular Linux binaries in them.

Ed: for example, AFAIK there's no native build of Microsoft sql server for freeBSD, but there's a release for Linux, and a docker (oci) image:

https://hub.docker.com/_/microsoft-mssql-server

So, assuming runj could fetch from docker hub, and interpret the images, you might in theory just:

runj run mcr.microsoft.com/mssql/server

(I'm not sure about the status of init vs oci - I know docker defaults to/uses tini these days?).