Docker watcher: Port is required?

Sorry for this being rather a question than an issue… the Docker watcher requires a port for each service if I understand the code right. Wouldn't it make sense to have the watcher just look for an image-tag combination and figure out the port (or ports if there are several) by itself? That way one could spin up a new deploy with a "staging" tag, test drive it, then add a "production" tag and take down the old deploy. Or am I missing something?

The issue does not name a file, test, or entry point. Start by tracing the Docker watcher’s port requirement and how it identifies image-tag combinations; then determine whether automatic discovery of one or more ports can support the proposed staging and production workflow and define concrete acceptance criteria.

Written by the indexing model from the issue text.