add aspnet-mssql application sample

Signed-off-by: Anca Iordache <anca.iordache@docker.com>

samples/aspnet-mssql/backend/Dockerfile.preview

@@ -0,0 +1,19 @@
+# This Dockerfile uses nightly preview builds for .NET Core
+FROM mcr.microsoft.com/dotnet/core/sdk:2.2 AS build
+WORKDIR /app
+
+# copy csproj and restore as distinct layers
+COPY *.sln .
+COPY aspnetapp/*.csproj ./aspnetapp/
+RUN dotnet restore
+
+# copy everything else and build app
+COPY aspnetapp/. ./aspnetapp/
+WORKDIR /app/aspnetapp
+RUN dotnet publish -c Release -o out
+
+
+FROM mcr.microsoft.com/dotnet/core/aspnet:2.2 AS runtime
+WORKDIR /app
+COPY --from=build /app/aspnetapp/out ./
+ENTRYPOINT ["dotnet", "aspnetapp.dll"]