Why does aspnet core start on port 80 from within Docker?

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The microsoft/aspnetcore-build container builds on top of the
microsoft/aspnetcore container. The dockerhub page for that says:

A note on ports

This image sets the ASPNETCORE_URLS environment variable to http://+:80 which means that if you have not explicity set a URL in your application, via app.UseUrl in your Program.cs for example, then your application will be listening on port 80 inside the container.

So this is the container actively setting the port to 80. You can override it, if you want, by doing this in your Dockerfile:

ENV ASPNETCORE_URLS=http://+:5000

Also, it is worth noting that because of the docker command you are using, you will still be able to access the application at http://localhost:5000 whether you are running the application directly or in a container.

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