A tarball of tarballs, layered using barely documented rules (e.g. you can add a directory, remove it on upper layer, and re-add something else on top).
Constructing them is easier than unpacking. :)
Constructing them is easier than unpacking. :)
I'm definitely ignorant of the exact rules for those; I only use them to split up large images into a "mostly static" layer (e.g. dependencies, which may be large) and a "subject to change" layer (i.e. our code, which will be different for each version)