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Build your ASP.NET Core application outside the Docker container

Most of the examples you find when using an ASP.NET Core application inside a Docker container use the multistaged build approach.

In this approach you create a dockerfile where building the application happens inside the docker file, the output of this build will then be used in a second stage to create the final docker image:

This is fine and results in small optimized docker images. So why this blog post?

The problem becomes clear when you take a look at our build pipeline:

What you can see is that we build the application twice; one time to run the unit tests, code analysis, vulnerability scanning etc… and one time to produce the docker image. Although we use different stages in Azure pipelines, it is still a waste of resources.

An alternative approach is to build your ASP.NET Core application outside the Docker container. The dockerfile is only used to copy the build artifacts from the publish folder into the docker image.

More information: https://docs.microsoft.com/en-us/dotnet/core/docker/build-container?tabs=windows#create-the-dockerfile

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