What's the correct Windows Base Image for ASP.NET Core 2.1 Docker Containers on Azure App Services

Question

I want to create a Docker Image that can be hosted on Azure App Services for Windows. My application is based on ASP.NET Core 2.1 and according to the official list of .NET images images, I should be able to simply use microsoft/dotnet:2.1-aspnetcore-runtime.

I can build the Dockerfile on a Windows machine successfully and was able to run it there without issues. But after uploading it to Docker Hub and setting it as the App Service's Docker Image, I get the following error message:

Cannot run this Operating System/Version in Windows Containers. Maximum supported OS version is 10.0.14393.9999.

According to the Azure App Services Documentation, it should support the microsoft/dotnet:2.1-aspnetcore-runtime as one of the pre-installed parent images.

When inspecting my Docker Image, I found, that the used image seems to be too new for Azure App Services:

"Architecture": "amd64",
"Os": "windows",
"OsVersion": "10.0.17134.285" <-- too new

After some research, I found in this blog post, that Azure App Services on Windows might only accept microsoft/dotnet:2.1-aspnetcore-runtime-nanoserver-sac2016 image. So I tried to rebuild the Docker Image with these.

This time, the App Service accepted the image, but was unable to start it, throwing the following logs:

02/10/2018 14:15:09.437 ERROR - Site: rothiewindockerdemo - Image pull reported error. Image: robinmanuelthiel/contosomaintenance-api:latest-windows-sac2016. failed to register layer: re-exec error: exit status 1: output: remove \\?\C:\DockerData\windowsfilter\93b716197958ceb58006ff3d978fcb3202f7866d00d6d8d69513cf0478a17a7f\UtilityVM\Files\Windows\servicing\Packages\Microsoft-UtilityVM-Core-Package~31bf3856ad364e35~amd64~~10.0.14393.0.cat: The process cannot access the file because it is being used by another process.
02/10/2018 14:15:09.437 INFO - Site: rothiewindockerdemo - Image: robinmanuelthiel/contosomaintenance-api:latest-windows-sac2016
Custom Registry: https://index.docker.io

02/10/2018 14:15:09.439 ERROR - Site: rothiewindockerdemo - Pull image completed but it was not found locally. Image: robinmanuelthiel/contosomaintenance-api:latest-windows-sac2016
02/10/2018 14:15:09.441 WARNING - Site: rothiewindockerdemo - Attempt 1 to start container was unsuccessful. Maximum attempts: 3. 
02/10/2018 14:15:09.568 INFO - Site: rothiewindockerdemo - Purging after container failed to start
02/10/2018 14:15:09.582 INFO - Site: rothiewindockerdemo - Purging pending logs after stopping container

So what is the correct Windows Docker Base Image for ASP.NET Core 2.1 Docker Containers on Azure App Services?

That's my Dockerfile:

#######################################################
# Step 1: Build the application in a container        #
#######################################################

# Download the official ASP.NET Core SDK image 
# to build the project while creating the docker image
FROM microsoft/dotnet:2.1-sdk as build
WORKDIR /app

# Restore NuGet packages
COPY *.csproj .
RUN dotnet restore

# Copy the rest of the files over
COPY . .

# Build the application
RUN dotnet publish --output /out/ --configuration Release

#######################################################
# Step 2: Run the build outcome in a container        #
#######################################################

# Download the official ASP.NET Core Runtime image
# to run the compiled application
FROM microsoft/dotnet:2.1-aspnetcore-runtime

WORKDIR /app

# Open HTTP and HTTPS ports
EXPOSE 80
EXPOSE 443

# Copy the build output from the SDK image
COPY --from=build /out .

# Start the application
ENTRYPOINT ["dotnet", "MyApp.dll"]

Answer 1

The issue lies in the fact that microsoft/dotnet:2.1-aspnetcore-runtime is a multi-architecture base image. This means that Docker build will pick the best architecture for your local machine (the machine where you are building your docker image). I assume that your local machine is Windows 10 April 2018 Update (Version 1803 - Whose build number 17134.407). As of now, we only support images based off of Windows Server 2016 (Version 1709, up to build number 14393.XX).

In order to “force” a specific version, please use this base image instead: microsoft/dotnet:2.1-aspnetcore-runtime-nanoserver-sac2016. You can check all the available tags in https://hub.docker.com/r/microsoft/dotnet/

We will work on calling this out specifically in our documentation.