There's a continuum here between "stay compatible with existing code" and "do things in a new and interesting way". Docker stays firmly on the compatibility end, and obviously for people wanting to use software that exists, that's a great thing. Sandstorm tries to strike a balance by making it relatively easy to port existing apps, but also trying to do things pretty differently, scaling things so that every document gets its own independent container and the only way to talk to the rest of the world is through capability-based Cap'n Proto RPC. It will take longer to populate the ecosystem, but apps will be able to do different kinds of things.

Sandstorm does use the same underlying kernel features as Docker, so any advances there should benefit both. Above that, I'm not actually sure if trying to share userspace tools would benefit either project, given the very different design decisions. It might just become a tug-of-war, like Chrome and Safari sharing Webkit while going in different directions with their sandboxing model.