Skaffold push always HTTPS despite insecure-registry flags

As part of a CI pipeline, I'm running a local registry, which is by default insecure:
`docker run -d --name registry.localhost -v local_registry:/var/lib/registry --rm -p 5000:5000 registry:2`

I'm definitely able to resolve the registry (eg my /etc/hosts mappings are all good), however passing:
```
SKAFFOLD_DEFAULT_REPO="registry.localhost:5000" skaffold build --insecure-registry=registry.localhost --insecure-registry=registry.localhost:5000 --cache-artifacts=false
```

Gives me:
```
couldn't build "blah": could not push image "registry.localhost:5000/blah:7d38d94d5b9-dirty": Get https://registry.localhost:5000/v2/: Service Unavailable
```

### Expected behavior

At the very least, I'd have expected it to try and push to the http endpoint - *http*://registry.localhost:5000/v2

### Actual behavior

It tries to push to *https*://registry.localhost:5000/v2

### Information

- Skaffold version: v1.10.1
- Operating system: OSX

What might I be missing here? I've also tried the environment variable way of specifying the insecure registries. Does this only work with `skaffold dev` perhaps?