@NobbZ I just spent a day building a phoenix release using nix with the goal to build a minimal docker image.
I found the same problems you mention here with erlang and systemd being included leading to massive docker images.
Are you saying, that even if you worked around these problems your image size is still 150MiB?
Nix has a lot of promise but, working through the somewhat hard to read documentation to finally manage to build a phoenix derivation and create a docker image only to see such a substandard result was pretty disappointing.
I wish I had seen your post before I started rather than when I was frantically trying to trouble-shoot the problem