rucio.image.registry does not work for those who build their own rucio images

[ericvaandering]

The rucio.image.registry setting would be useful for us to get things like busybox, fluentbit, etc from a mirror. However, we don't put our own rucio images in docker.

The easiest thing to do would be to split these settings into rucio.image.registry and perhaps docker.image.registry or some other name. Then people could set one or both of them.