Define a GC policy for the node images

[jlebon]

IIUC ghrc might not be GC'ing images by default. Let's make sure that happens. Perhaps e.g. keep the last 30 days?

Reusable code from chunkah: https://github.com/coreos/chunkah/blob/26dfa025088c7ef678628753397310a86f9dbc91/.github/workflows/ci.yml#L196-L203