[Docker-maint] Removal of docker.io from jessie

[Florian Weimer]

The security team has concerns that docker.io cannot be maintained in
jessie.

I asked upstream about the Go version commitment (we cannot rebase to
Go 1.4 or later in jessie because it could break user code):

  <https://groups.google.com/forum/#!topic/docker-dev/BjNmlgifZ5c>

(Not sure if this link will work, it obviously requires Javascript.
It should point to the docker-dev thread, “Go version requirement”,
started on 2015-03-15.)

I think the Go version issue has been adequately addressed, but other
parts of the thread show that there is no clear plan how to rebase
docker.io to a new upstream version once this becomes necessary.
Hence our concerns about maintainability.

(The rebase question is not entirely theoretical.  Upstream has
previously acknowledged that “the v1 registry has a flawed design”:

  <https://news.ycombinator.com/item?id=8789775>

The v1 registry protocol is what is implemented in docker.io 1.3.3.
Debian does not run its own trusted registry, so our users are fully
exposed to these design issues.)