This sounds promising @zimbatm.
Any ideas of a minimal derivation that would create such an image (with nix tooling available at /bootstrap/nix and store realised to bind mounted /nix/store)?
We’re facing this issue now attempting to use the nixos/nix image that already ships with a /nix/store and attempting to bind a persistent (and so far blank) /nix/store.
Alternatively, (as @paralin mentioned) I wonder how to make use of the overlayfs-driver to layer the stores, treating the container’s /nix/store as read-only, but the bound /nix/store as read/write?