Support private Docker runner images on the public worker pool

Allow stacks using the public (shared) worker pool to pull runner images from private container registries (e.g. ECR, Docker Hub private, GCR), rather than requiring images to be publicly accessible.

Currently, customers on the public worker pool must publish their runner images to a public registry.  With how docker images currently work, this is intentional as the Docker daemon is shared across tenants, so cached image layers could be accessible to other customers. Private worker pools don't have this constraint since the user owns the infrastructure. (

https://docs.spacelift.io/integrations/docker.html#using-private-docker-images)

But in the user’s situation they would rather accept the risk of their image being leaked via the Docker daemon to other Spacelift customers rather than having to publish the image to a public registry.