Azure web app deployed as docker container endless 301 loop - asp.net-core

Question

We are trying to migrate our REST Web API from being hosted as a Windows .NET Core 3.1 stacked web app to a containerized web app on Linux on Azure.
So far we have managed to push the image to the Azure Container Registry where it's being automatically picked up and successfully deployed to an App Service. Unfortunately, the app does not properly work yet. When trying to fetch some configuration data from a (anonymous) end-point from our API (GET https://foo.azurewebsites.net/api/configuration), instead of returning the data - as it used to do - I get a 301 (Moved Permanently) status code that points exactly to itself: location: https://foo.azurewebsites.net/api/configuration which leads to redirection loop.
So far I have no idea why I'm getting a 301 and I'm glad for any hints.
Points of interest:
Docker: the base for the image is: mcr.microsoft.com/dotnet/core/aspnet:3.1
Azure: Authentication / Authorization is switched off
Azure: no Front Doors are installed
The app is correctly serving the Swagger UI.
The Docker image works fine locally.

Answer 1

Here's how I solved the problem: it turned out that the cause for the permanent redirect loop was a conjunction of how the proxy works in the Azure deployment (thanks to Jason Pan for pointing me in that direction) and the following code that we had in our Startup:
services.AddControllersWithViews()
.AddMvcOptions(o =>
{
...
o.Filters.Add(new RequireHttpsAttribute { Permanent = true }); // REMOVE THIS LINE
...
});
Once I removed RequireHttpsAttribute filter, the app started working as expected. And since I've configured the TLS/SSL settings to allow HTTPS only, I think it is safe to omit the filter.
UPDATE 2021-01-20
I've just figured out that there's a better way to do this that does not require to remove the RequireHttpsAttribute filter. The core of the problem is that Kestrel does not know that communication is happening over a secure channel as the reverse proxy is forwarding requests over http to Kestrel. So we need to enable the forwarding of headers. For .NET Core 2.x applications this meant to follow the steps explained in Configure ASP.NET Core to work with proxy servers and load balancers. Luckily, for ASP.NET Core 3.x applications there a much more simpler way (that unfortunately is not mentioned in the official docs yet but was part of the preview 6 announcement): simply set the ASPNETCORE_FORWARDEDHEADERS_ENABLED environment variable to true. This can be done the usual way in the Azure portal under Confguration > Application settings: