Dotnet docker experiences

on Regards: Infrastructure; Docker; Azure;

Transitioning an Application from On-Premises to Azure Docker Containers

In a previous blog post, I discussed the process of migrating an on-premises application to an Azure Scale Set. Recently, I had again the opportunity to transition an existing .NET background service to Azure using docker. A key distinction between the previously migrated application and the one targeted for this migration was its lack of Windows dependencies. This meant that the application could be feasibly migrated to a Docker container.

Starting with docker containers

Despite my limited experience with Docker containers, I was eager to delve deeper and expand my knowledge. I immersed myself in numerous documentation resources to understand how to create a Docker container using a Dockerfile. Most examples demonstrated the use of a Linux base image and the building of the application using the .NET CLI like the following:

FROM AS build-env

# Copy everything
COPY . ./
# Restore as distinct layers
RUN dotnet restore
# Build and publish a release
RUN dotnet publish -c Release -o out

# Build runtime image
COPY --from=build-env /App/out .
ENTRYPOINT ["dotnet", "DotNet.Docker.dll"]

The private nuget feed problem

The above Dockerfile works great for applications that do not have dependencies on private nuget feeds. However, as you might have anticipated, the application targeted for migration did rely on a private NuGet feed. One potential workaround for this predicament would be to incorporate the credentials for the private NuGet feed directly into the Dockerfile. There are diffrent ways to accomplish this, one of them can be read here Consuming private NuGet feeds from a Dockerfile in a secure and DevOps friendly manner.

While this approach might seem straightforward, it’s not without its drawbacks. Apart from the glaring security implications, the process of making these adjustments can be quite cumbersome.

Fortunately, the .NET CLI comes to the rescue, offering support for building Docker images with the command dotnet publish --os linux --arch x64 /t:PublishContainer -c Release. The Docker image was constructed within a CI/CD pipeline, an environment already authenticated and granted access to the private NuGet feed. As a result there was no need for additional configuration.


      - task: DotNetCoreCLI@2
        displayName: 'dotnet restore'
          command: 'restore'
          projects: '*.sln'
          feedsToUse: 'config'
          nugetConfigPath: '$(Build.SourcesDirectory)/nuget.config'
      - task: DotNetCoreCLI@2
        displayName: 'dotnet build'
        condition: succeeded()
          command: 'build'
      - task: DotNetCoreCLI@2
        displayName: 'dotnet publish'
          command: 'publish'
          arguments: '-c Release --os linux --arch x64 /t:PublishContainer /p:ContainerImageTag=latest /p:ContainerRepository=DotNetDockerTemp'
          projects: $(Build.SourcesDirectory)/src/**/DotNetDockerTemp.csproj
          feedsToUse: 'config'
          nugetConfigPath: '$(Build.SourcesDirectory)/nuget.config'

      - task: PowerShell@2
          targetType: 'inline'
          script: |
            docker save DotNetDockerTemp:latest -o $(Build.ArtifactStagingDirectory)/DotNetDockerTemp.tar

      - task: PublishBuildArtifacts@1

(1. Code snippet from the build pipeline)

Please notice if you want to create a docker image based on the alpine image, you need to specify the correct runtime identifier (RID) for the alpine image, otherwise you will not be able to start the application. You can find the RIDs here: RID catalog and more about the issue here.

dotnet publish -c Release --runtime=linux-musl-x64 /t:PublishContainer /p:ContainerImageTag=latest /p:ContainerRepository=DotNetDockerTemp /'

Multiple Enviroments and the appsettings.json

You may have noticed in the above build pipeline that we are using the dotnet publish /t:PublishContainer command. This command builds and publishes the Docker image to the local Docker repository. While this is excellent for local development, it raises questions about handling multiple environments and the appsettings.json file.

To address this, we save the local published Docker image into the DotNetDockerTemp.tar file and add it to the artifacts of the build pipeline (see 1. Code snippet from the build pipeline). During the release pipeline, we download the artifact and load the Docker image into the local Docker repository using the docker load -i DotNetDockerTemp.tar command.

Next, we tackle the appsettings.json file. For this, we require the FileTransform@1 task, which transforms the appsettings.json file based on the environment. While we can’t inject the appsettings.json file into the existing Docker image DotNetDockerTemp:latest, we can create a new Docker image based on the existing one.


  - task: PowerShell@2
    displayName: "Inject" appsettings.json into docker image
      targetType: "inline"
      script: |
       docker create --name dockertemp DotNetDockerTemp:latest

       docker cp appsettings.json dockertemp:/app/appsettings.json     
       docker commit dockertemp DotNetDockerFinal:latest

  - task: AzureCLI@2
    displayName: Azure Container Registry Login
      scriptType: 'pscore'
      scriptLocation: 'inlineScript'
      inlineScript: |
        az acr login --name $(AzureContainerRegistryLoginName)
        docker push [...]
  1. Code snippet from the release pipeline

Finally, as a last step, we could push the docker image to the Azure Container Registry.

Custom DNS and SSL Certificates

Upon starting the Azure Container Instance (ACI), I discovered that the service was unable to access certain endpoints. After conducting some investigations, connecting to the container instance with az container exec (or using the azure portal) and probing with nslookup and related commands, I deduced that the Docker container instance was incapable of resolving our custom domain names. This is understandable, as the Docker container instance does not know our custom DNS server. This issue can be resolved by configuring the dnsConfig for your ACI.

With the updated settings, the app was able to resolve the endpoint but encountered an exception due to an invalid SSL certificate. Once again, this is logical as the Docker container instance is unaware of our custom SSL certificate. To enable the app or the operating system to accept the custom certificate of our endpoint, we need to add the public key of the certificate to the trusted root certificates of the Docker container instance. I was unable to find a method to accomplish this within the pipeline using the Docker command line. Consequently, I opted to create a custom Docker image based on the existing one and add the public key of the certificate to the trusted root certificates of the Docker container instance.

FROM DotNetDockerTemp:latest AS base

# Second stage: Use the Alpine image
FROM AS final

# Copy files from the first stage
COPY --from=base /app /app
USER root

COPY ["certificate.crt", "/usr/local/share/ca-certificates/"]
RUN apk add --no-cache ca-certificates
RUN update-ca-certificates


ENTRYPOINT ["/app/DotNetDocker"]
CMD ["-s"]

This dockerfile use the docker image which was previously created with the dotnet publish /t:PublishContainer [...] command and add the public key of the certificate to the trusted root certificates of the docker container instance. Since the app DotNetDocker needs to be started with DotNetDocker -s we need to add the -s to the CMD command. To build the docker image using the dockerfile we can use the following command:

docker buildx build -f src/Dockerfile . -t localhost:5000/DotNetDockerFinal:latest --no-cache --progress=plain
  • -f: Path to the dockerfile
  • .: Path to the context; current directory where docker buildx build is executed (important for the COPY command)
  • -t: Tag of the docker image
  • –no-cache: Do not use cache when building the image
  • –progress=plain: Show the progress of the build

Wrapping Up

Transitioning the application into a Docker container proved to be a swift and straightforward process. However, it’s important to note that this journey can entail numerous considerations that may demand a significant investment of time, particularly when the source code of the application is not open to modifications.

I’m eager to hear your thoughts on this process. Would you have approached it differently? Do you have any queries or insights to share? Your feedback is invaluable, and I look forward to our continued discussions on this topic.