Fix Docker build with new D.NET

Dockerfile

@@ -2,9 +2,8 @@ FROM mcr.microsoft.com/dotnet/core/sdk:3.1-alpine AS build
 
 WORKDIR /app
 COPY . /app
-RUN dotnet publish -c Release -o out
+RUN dotnet publish -c Release -o out -f netcoreapp3.1
 
-# TODO: is using aspnet correct here? Required for API but might break Bot
 FROM mcr.microsoft.com/dotnet/core/sdk:3.1-alpine
 WORKDIR /app
 RUN dotnet tool install --global dotnet-trace && dotnet tool install --global dotnet-dump && dotnet tool install --global dotnet-counters