Support --runtime for docker image

[djr7C4]

I sometimes use runsc (gVisor) as the runtime but it's currently not supported. I've locally added --runtime to the transient with completing-read to select one of the installed runtimes. Let me know if you'd be interested in a PR.

[Silex]

Well I don't use it (and probably few do) but if you think there's value for this package, sure make a PR :-)

[djr7C4]

Fixed by #256.