[Bug] Pulling image is now based on push_time which is not updated if image is identical

[bartvanb]

Describe the bug
In pull_if_newer(), we pull the image if the remote timestamp is newer than the local one. Recently, we changed this (#482), and based the timestamp on the push time.

It can occur that the local and remote image are identical, but that the remote push timestamp is newer. Then, the 'newer' remote image is pulled over and over again without ever updating the timestamp. This should be achieved somehow so that no pulls are attempted.

Note that when the images are identical, the pull time is short, so this issue is not very pressing for that reason.