allow custom external image

[MasslessParticle]

This change adds a property that allows the user to configure a non-standard image to be used as the node-image.

We needed to pull a node image from a corporate proxy to avoid docker rate limiting so building our own via the node-input property didn't make a ton on sense.

Readme formatting got kind of mangled, sorry.

[hoegaarden]

Sorry for the late reply!

I think the same can be achieved already, by pulling in your custom image via a concourse resource (registry-image, with format: oci) and feed that into kind-on-c via the node-image input.
I would rather prefer that, so concourse keeps track on which image is being used and the task container does not go off an pull in "random things" (it already does that too often, as it stands right now).

Would that solve your issue?