docker-registry/Dockerfile.template: Launch with 'exec docker-registry'

And modernize the installation and execution:

* Remove setup-configs.sh call, because the script is gone since
  2ee1387a (Clean up configuration management, 2014-05-08).

* Use the setup.py-installed docker-registry endpoint, because run.sh
  is gone since bf3360a2 (replacing run.sh with a "docker-registry"
  console script, 2014-04-02).  Unfortunately, I can't find a way to
  run docker-registry from the source directory (without installing
  it), because Setuptools' pkg_resources seems to need some
  namespace-package *.pth funkyness.  Luckily all this is easier with
  the PEP 420 namespace implementation that landed in Python 3.3.

* Don't pass '--preload' to Gunicorn, because docker_registry.run is
  loading bulk of docker_registry before launching the Gunicorn
  process.  The stack pulling in the initial database-setup is:

    File "/.../docker_registry/run.py", line 23, in <module>
      from .search import *  # noqa
    File "/.../docker_registry/search.py", line 17, in <module>
      INDEX = index.load(cfg.search_backend.lower())
    File "/.../docker_registry/lib/index/__init__.py", line 82, in load
      return db.SQLAlchemyIndex()
    ...

  It seems inefficient to drag all this into the Gunicorn process for
  folks that don't need single-threaded setup, but whatever, it works
  for my use case ;).