Currently if I specify a mount propagation flag in "options", it does not take effect. For example try following bind mount in config.json.

{
"type": "bind",
"source": "/root/mnt-source",
"destination": "/root/mnt-dest",
"options": "rbind,shared"
}

And inside container check the properties of mount.

$ findmnt -o TARGET,PROPAGATION /root/mnt-dest
/root/mnt-dest private

It continues to be private, despite the fact user requested for it to be "shared".