get the k3d local registry working with tilt

[clux]

supposedly this is pretty easy. made a registry node for my k3d, but it still pushed to my dockerhub. might need to create the cluster with the either --registry-create or --registry-use.

anyway want the tilt up experience working for users, and currently it won't because it needs to push to my dockerhub.

• try to follow https://k3d.io/usage/guides/registries/#using-a-local-registry and make it work
• link to that guide in the readme (we already mention the local registry)
• keep it simple enough so that we don't have to pollute the readme in this repo with linux specifics
• some fallback mechanic if they don't have a k3d cluster?
• test with k3d 5

suggestions welcome, coming back to this at some point.