Update Dockerfile with MCR images (#276)

Author: dlepow

apis/poi/web/Dockerfile

@@ -1,4 +1,4 @@
-FROM microsoft/dotnet:2.1-sdk AS build-env
+FROM mcr.microsoft.com/dotnet/core/sdk:2.1 AS build-env
 WORKDIR /app
 
 # copy csproj and restore as distinct layers
@@ -12,7 +12,7 @@ COPY ./appsettings.*.json /app/out/
 COPY ./appsettings.json /app/out/
 
 # build runtime image
-FROM microsoft/dotnet:2.1-aspnetcore-runtime
+FROM mcr.microsoft.com/dotnet/aspnet:2.1
 WORKDIR /app
 
 ENV SQL_USER="YourUserName" \