Prepare for running apps and localtest with test containers for integration testing

[ivarne]

I would consider this a cleanup PR with a few changes to localtest that would simplify using localtest for integration testing in testcontainers.

• Set ASPNETCORE_URLS=http://*:5101/ in dockerfile Localtest should always run on port 5101, so it is better to specify in Dockerfile than at runtime
• Add proxy middleware so that localtest can be used without nginx "loadbalancer" Running NGINX is hard, because it crashes if one of the backing servers is unavailable at startup. This simple dotnet proxy ensures that the client can have both app and localtest on a single domain:port.
• Use Request.Host instead of env config for openId-configuration The app will access localtest on the internal docker network, and the test runtime accesses via port mapping, so we need some sort of logic.

[martinothamar]

Generally LGTM. Can we use YARP for the proxying instead? I've tried something like this before as well and dicovered quickly there is more nuance. YARP was pretty well-abstracted last I tried, it was possible to use as a simple library: https://github.com/dotnet/yarp/blob/41e563cced47925142bd5c6ddeb1e2958e0ebabd/src/ReverseProxy/Forwarder/HttpForwarder.cs#L27