The linux-tegra mailing list is the canonical communication channel when it comes to upstream Linux kernel development.
I should add here that both Alex and I focus almost exclusively on upstream development, which may or may not be what you're looking for. On one hand, upstream is where most of the new technologies are developed, so it's the "bleeding edge" from that point of view. On the other hand the amount of manpower that goes into upstream development is limited, so the upstream kernel might lack a bunch of features that you might be interested in. That said, we are working very hard on getting upstream closer to L4T/Android from a feature perspective and would very much welcome contributors.
We also have an IRC channel (#tegra on Freenode) that we use for realtime communication. If you're interested in chatting with us, feel free to drop by.
Not exactly sure where they differ. But the most obvious thing is that Snappy has a completely different way of installing packages and its own package format, too.
HypriotOS on the other hand just uses the standard Debian way of installing and managing software.
Everybody who is interested in test-driving Docker 1.6 in a really easy and fast way and has a Raspberry Pi lying around should have a look at our prepared Docker SD card image.
Not sure if images on the registry also support the new labels functionality, but that might be a way to avoid to obvious image architecture (x86 vs ARM) issues.
Currently this is more of an experiment to see if it is possible to get Docker running. The image is based on Raspbian with a custom kernel compiled for working with Docker and OverlayFS. We will publish our buildscript within a couple of days on github. Based on the feedback we get we want to provide updates to the image on a regular base.
Thanks. I would very much like to see that buildscript - I've been tinkering with the idea of building something along these lines but with an Ubuntu armhf userland (given that Ubuntu on ARM has a lot of advantages for me).