allow using different varnish image, also different versions, maybe the upstream image

[ibotty]

It would be great to allow a different image to be specified.

Also can it be documented what has to be customized ATM.

[cin]

The docs for updating the varnish image (and other images) are here. You should be able to update the varnish.image property but be careful moving to different varnish versions because the operator may not work with the different versions' APIs/CLIs. An example is here. HTH.