> dev environments or if they're developing on their host

It's usable for both. Some people go one way some the other. It definitely makes developing in a container on a mac easier if you don't have to worry about compatibility with the target environment.

> If you use a container for only your post-development workflow do you ever use separate containers for build and test vs the final packaged application?

Yes, most of the time, mostly because the tests require extra dependencies which are not needed in prod.