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

without dockerfile you can set any port out of the docker container. (.NET Core 3.1, .NET 5, .NET 6, .NET 7+) with docker args

docker run -it --rm -p 5000:80 -p 5001:443 -e ASPNETCORE_HTTPS_PORT=https://+:5001
           -e ASPNETCORE_URLS=http://+:5000 --name aspnetcore_sample aspnetapp

more details:

https://github.com/dotnet/dotnet-docker/blob/17c1eec582e84ba9cbea5641cd9cc13fe1a41c39/samples/run-aspnetcore-https-development.md?plain=1#L85

https://github.com/dotnet/dotnet-docker/blob/5926a01d44bd47b6202ba71e30f9faa08fad1aec/samples/run-in-sdk-container.md?plain=1#L109

If you are using .NET Core 2.2 or higher, then you should to use another image: mcr.microsoft.com/dotnet/core/aspnet:2.2. In that case specifying ENV ASPNETCORE_URLS=http://+:5000 does not help. You still can force app to listen to port 5000 by using UseUrls("http://*:5000") in Programs.cs file.

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.