I generally do my development in an image built from the latest 7.1 branch. This had a commit in July "New method category chunk format to reduce 7<->8 merge conflicts" (see issue 1138) which prevents packages saved from 7.1 images built after this commit from loading in earlier images. The 7.1.20 release pre-dates this, so can't load more recent packages.
The solution is to use a more recent image; it's probably also worth raising an issue asking for the 7.1 release to be updated.
Hope this helps.