Injecting code into a compiled, proprietary DotCore application

My question is literally this question:

How can I use IdentityServer4 from inside and outside a docker machine?

Except I do not have the source to the IdentityServer. Using DotPeek I can see this is missing:

services.AddIdentityServer(x =>
{
    x.IssuerUri = "my_auth";
})

That it does not expose IssuerUri which seems to be the issue. For some reason I cannot get the compiledproject to recompile. Is there anyway I can inject that piece of code? Very frustrating that all I need to do is to do that and I can get my docker swarm working happily. When I export it from DotPeak there's some things that don't compile right.