Juniper provides a tarball to import into Docker:
docker image load -i junos-routing-crpd-docker-19.2R1.8.tgz
To get a crpd.img:
container=$(docker create crpd:19.2R1.8)
virt-make-fs --partition --format=qcow2 =(docker container export $container) crpd.img
docker container rm $container