Cannot reach web server inside Docker

I’m doing some recapitulation of my Docker knowledge but have run into a problem that I don’t understand.

I created a simple web server in Go and have no problems running it inside a Docker container. But I cannot reach it from outside.

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    You can see my Dockerfile and my small Go code in this gist:
    https://gist.github.com/md2perpe/2639afcd79e359a857ef94ba7908fb80

    Why can’t I reach the small web server?

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