Where is 'localhost'? (Docker networking)

Building up to the devproxy2 release, I set things up to test in Docker, using a known version of Go.  Nothing worked for quite some time.

Core problem: localhost inside the container is not the host network.  I had to configure it to listen on all addresses (by asking Go to connect to ":8080" without a host address), per this other Stack Overflow answer. If I had thought to check the exact error message client side (curl: empty reply from server), I could have solved this one first, not last.

Once I fixed the core problem, everything started working, and I didn’t go back and test the other things I had tried.  So everything else is only a potential problem.

Potential problem: localhost inside the container is not the host network.  I configured it to connect to host.docker.internal instead, per this Stack Overflow answer.  This is the same as the core problem, but in the other direction.

Potential problem: according to my notes, the default network bridge in Docker doesn’t have Internet access. Creating a user-defined bridge solves this.  Furthermore, using Docker Compose automatically creates a user-defined bridge for the stack.  By the time I solved the core problem, I was already using Docker Compose for the networking feature.