How can I use docker to develop with 0.4.0? (i.e., how can I configure where /_build/ is exposed?)

[knpwrs]

I use docker for my local development, and I have the primary application port exposed on my machine. Since upgrading to 0.4.0, however, the network tab shows repeated attempts to /_build/ on a different port than what the rest of the app is served from:

How can I configure the port where /_build/ is served from so I can reliably expose it from Docker and the connection can succeed? I've tried tracing the code through to vinxi but wasn't able to determine the correct parameters for my configuration.