This is a don't forget issue related to #19116 and the --rm case. I'll quote @WeiZhang555

Also today I may find another problem, if we run

$ docker run --rm --restart=always busybox top
Conflicting options: --restart and --rm

So it forbid me from using --restart with --rm. Then what will happen if run

$ docker run --rm busybox top
$ docker update --restart=always xxxx

I guess there will be something broken, I haven't checked it yet, later I will make a more detailed investigation.

The idea would be to kinda let know the daemon that the container is gonna be removed at some point and thus, disallow updating the restart policies. Not sure this is the right solution, but we need to think of it.