added docker file for sample echo server

Author: kerryjiang

.dockerignore

@@ -0,0 +1,2 @@
+bin
+obj

samples/EchoServer/Dockerfile

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+FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build-env
+WORKDIR /app
+
+COPY . ./
+WORKDIR /app/samples/EchoServer
+
+RUN dotnet publish -c Release -o /app/out -f net8.0
+
+
+# Build runtime image
+FROM mcr.microsoft.com/dotnet/aspnet:8.0
+
+WORKDIR /app
+COPY --from=build-env /app/out .
+
+ENV ASPNETCORE_URLS http://*:5000
+ENTRYPOINT ["dotnet", "EchoServer.dll"]