A fatal error was encountered. The library 'libhostpolicy.so' required
After some googleing I found this:
https://github.com/aspnet/aspnet-docker/issues/301
I'm not certain what caused it but it seems to have gone away. It's probably because I had a case mismatch of appSettings.json instead of appsettings.json which meant that the file was not being loaded. Terrible error though.
In my case it was that I had forgot to put appsettings.json to copy on builds.
Hope this helps someone else and get's higher on google searches.
After even more googling, we found this link:
http://szahariev.blogspot.co.nz/2016/10/libhostpolicyso-was-not-found-error.html
which lead us to discover that we'd somehow managed to manually remove the (in our case) <OutputType>Exe</OutputType> line from our .csproj file. So no runtimeconfig.json was created at all.