[Nix-dev] unequal build hash

[Harmen]

Hello all,
another day, another problem :/

I'm trying to figure out why a build generates different build IDs in different
contexts.
I have a nix expression and some code. If I copy that from machine A
to machine B they both give the same build hash. All's fine there.
If on machine B I run nix-build in a docker container it won't give the same
build hash.
`nix-hash .` gives the same hash for B and B-docker. Same nix version, same
nixpkgs version.

Any hints what to look for?
Thanks!