[rcoveson]

IMO image definitions should be a list of mounts that may be overlays on root but may also be more “normal” mounts to directories within root. I should be able to make an image that is ubuntu:bionic plus a conda installation at /opt/conda plus a personal package at /usr/local/mything. Currently you have to decide on how to stack those layers, which is unnatural and prevent sharing/deduplication of partial-file system images where there’s no reason to prevent it.

Taken to the extreme, look at something like Nix (or conda, come to think of it). Why can’t I just have one copy of a package of a given version shared by all containers, if they all want that package? Unix file systems should be great at that kind of composibility; that’s the advantage of a unified tree instead of a tree-per-source. But in the docker model, you’re stuck with a stack.

My ideal image definition is a hybrid between docker’s immutable hash-addressed image layers and an fstab file to describe how and where to mount them all.