Running Docker container fails due to dotnet-isolated not found

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Summary

The issue at hand is that a Docker container fails to run due to a dotnet-isolated not found error when trying to start an Azure Functions app made with .NET 8. This error occurs when building and running the image locally, despite deploying fine to Azure via Azure Container Registry.

Root Cause

The root cause of this issue is related to the FUNCTIONS_WORKER_RUNTIME environment variable, specifically when its value is set to “dotnet-isolated”. This variable is used by Azure Functions to determine the runtime to use for the application.

Why This Happens in Real Systems

This issue can occur in real systems due to the following reasons:

  • Incorrect or missing environment variables when running the container locally
  • Mismatch between the Dockerfile and docker-compose configurations
  • Inconsistent .NET versions or configurations between local and Azure environments

Real-World Impact

The impact of this issue includes:

  • Failed container startup, preventing the application from running locally
  • Difficulty in debugging, as the error message does not provide clear information about the cause
  • Inconsistent behavior between local and Azure environments, making it challenging to reproduce and fix issues

Example or Code

# Set the FUNCTIONS_WORKER_RUNTIME environment variable
ENV FUNCTIONS_WORKER_RUNTIME="dotnet-isolated"

# Run the Azure Functions app
dotnet run

How Senior Engineers Fix It

To resolve this issue, senior engineers would:

  • Verify that the Dockerfile and docker-compose configurations are correct and consistent
  • Check the environment variables set in the container and ensure they match the requirements of the application
  • Use debugging tools to investigate the issue and identify the root cause
  • Update the Dockerfile or docker-compose configurations as needed to fix the issue

Why Juniors Miss It

Junior engineers may miss this issue due to:

  • Lack of experience with Docker