Fix dockerfiles

Dockerfile.linuxamd64

@@ -1,4 +1,4 @@
-FROM microsoft/dotnet:2.1.505-sdk-alpine3.7 AS builder
+FROM mcr.microsoft.com/dotnet/core/sdk:2.1.505-alpine3.7 AS builder
 WORKDIR /source
 COPY NBXplorer/NBXplorer.csproj NBXplorer/NBXplorer.csproj
 # Cache some dependencies
@@ -7,7 +7,7 @@ COPY . .
 RUN cd NBXplorer && \
     dotnet publish --output /app/ --configuration Release
 
-FROM microsoft/dotnet:2.1.9-aspnetcore-runtime-alpine3.7
+FROM mcr.microsoft.com/dotnet/core/aspnet:2.1.9-alpine3.7
 WORKDIR /app
 
 RUN mkdir /datadir

Dockerfile.linuxarm32v7

@@ -1,5 +1,5 @@
 # This is a manifest image, will pull the image with the same arch as the builder machine
-FROM microsoft/dotnet:2.1.505-sdk AS builder
+FROM mcr.microsoft.com/dotnet/core/sdk:2.1.505 AS builder
 WORKDIR /source
 COPY NBXplorer/NBXplorer.csproj NBXplorer/NBXplorer.csproj
 # Cache some dependencies
@@ -9,7 +9,7 @@ RUN cd NBXplorer && \
     dotnet publish --output /app/ --configuration Release
 
 # Force the builder machine to take make an arm runtime image. This is fine as long as the builder does not run any program
-FROM microsoft/dotnet:2.1.9-aspnetcore-runtime-stretch-slim-arm32v7
+FROM mcr.microsoft.com/dotnet/core/aspnet:2.1.9-stretch-slim-arm32v7
 
 WORKDIR /datadir
 WORKDIR /app