Consider adding an equivalence of EXPOSE

Per https://docs.docker.com/engine/reference/builder/#expose:

> The EXPOSE instruction does not actually publish the port. It functions as a type of documentation between the person who builds the image and the person who runs the container, about which ports are intended to be published.

Some container service (like Azure App Service) consumes this metadata to automatically configure the correct port-mapping to the container. Without this metadata, users of these service will need to set this on the deployment side.

cc @jonjohnsonjr

Start with the linked Docker EXPOSE documentation to understand the image metadata and how services consume it. Then inspect ko's image-building flow to identify where equivalent metadata could be represented; done means the built image carries the intended port information for consumers such as Azure App Service, with behavior verified by appropriate tests.

Written by the indexing model from the issue text.