I just updated an application to Phoenix 1.5 following the instructions here: phx-1.5-upgrade.md · GitHub

Everything works beautifully in development mode, but when I build my (Docker) release the Endpoint doesn’t seem to start correctly, and won’t accept connections. It seems to be running, because if I try to start it I get this:

iex> MyAppWeb.Endpoint.start_link()
{:error, {:already_started, #PID<0.4364.0>}}

I never get the “Running MyAppWeb.Endpoint with cowboy 2.7.0 at 0.0.0.0:4000 (http)” log message, but I also don’t see any errors on startup. I’ve enabled debug logging in the release and didn’t see anything useful there. The release configuration is pretty much identical to a similar app using Phoenix 1.4 that works fine, including the server: true option, port configuration, etc.

Any ideas on how to debug this?