If there are accessories defined in the configuration, we'll not require
servers to be defined as well.
This allows for accessory-only configurations which allows you to run
external images with kamal-proxy for zero-downtime deployments.
We don't manage image cleanup for accessories though so the user will
need to deal with that themselves.
Use the --registry, --repository and --image_version options of
`kamal proxy boot_config set` to change the kamal-proxy image used.
We'll still insist that the image version is at least as high as the
minimum.