docker: set force=true on remove image to handle images referenced by multiple tags

[shoenig]

This PR changes our call of docker client RemoveImage() to RemoveImageExtended with
the Force=true option set. This fixes a bug where an image referenced by more than
one tag could never be garbage collected by Nomad. The Force option only applies to
stopped containers; it does not affect running workloads.

Fixes #15760

[tgross]

LGTM

Looking at the dockerCoordinator code, I suspect this will subvert the image_delay when folks have multiple tags on the same image if ref counts for those tags all go to zero within the delay window. But I also think if you care a lot about image_delay then you are going to care about canonicalizing which image tag you're using?

[jrasell]

LGTM!

[shoenig]

I suspect this will subvert the image_delay

Indeed, this swaps one bug for another, but I think for the less bad one.

With this change there's potential for missed optimization in re-using a downloaded image. Without, images can pile up on a node and require external maintenence as described in #15760. I added a note to the image_delay docs noting the caveat with referencing an image by multiple tags.