Run small Linux in RAM to provision a custom node image

We need to provision bare metal machines with custom node images. The bare metal machine provider has an API for booting the machine via iPXE. But no other APIs exist to provision a custom image. Current strategy: I think about starting a small Linux in RAM via iPXE which starts an ssh server. Then the custom infra controller could connect via SSH and run some scripts to download and install the node image on that machine. Up to now I did not find a suitable mini Linux for that. Most Linux systems are meant to be installed. [SystemRescue](https://www.system-rescue.org/) is quite big (almost 1 GByte). I would prefer a smaller tool. Building a custom initramfs might work, but using an existing tool is highly preferred. How would you solve that?