How to include a custom binary file in the published approot in .NET Core DNX?

Question

I'm developing an ASP.NET Core 1.0 application which I deploy to a Linux machine using Docker.

In my CI pipeline I'm publishing the project using dnu publish, then I build the docker image with docker build.

I have a static binary file that I need to use in my docker image. (It's a hotfix which I need to copy into the coreclr runtime directory.) I want to be able to access that file with a command from my Dockerfile.

What is the idiomatic, reliable way to make that file get copied into the output directory when I'm doing dnu publish? What I'm currently doing - because I couldn't find a better solution - is adding it to wwwroot, so it gets copied into wwwroot in the output folder.
However, I don't want that file to be publicly accessible, it would be much better to make it end up in approot.

The approaches I found on SO and tried, but are not working:

• Including the file in the project.json as "contentFiles": [ "mylib.so" ]. If I do this, I get the following error during the dnx build:
  Error: The process cannot access the file 'C:\myapp\src\myapp.web\mylib.so' because it is being used by another process.
• I tried to experiment with creating a custom postBuild script (found the suggestion here: New .NET "project.json" project, copying pre-built native .dlls to the output directory), but I can't figure out where to copy the file, since I have different CI pipelines publishing the same project using different publish output directories, so I cannot hardcode the directory path into my project.json.
  Or is there a way to copy it into the build directory, which will be picked up by dnu publish regardless of the output directory of publish?