You're right; that opening line was unduly harsh. My apologies. It's unfortunate, however, that the creator of the video did not capture the output of curl -v,

You're right; that opening line was unduly harsh. My apologies. It's unfortunate, however, that the creator of the video did not capture the output of curl -v, or perhaps even an strace. The logic presented in my first post is nonetheless what curl does, and should apply, so something else is going wrong here. I still think there's a wide difference between "in this particular case, something is causing the IPv6 address to hang" and "Localhost resolving to IPv6 basically breaks with Docker".