The docker VM server-side goo is mostly identical to Xen, with slightly
different handling for parent images.  We also support loading external
Docker images (i.e. those without a real imageid in our DB; in that
case, user has to set a specific stub image, and some extra per-vnode
metadata (a URI that points to a Docker registry/image repo/tag);
the Docker clientside handles the rest.

Emulab Docker images map to a Emulab imageid:version pretty seamlessly.
For instance, the Emulab `emulab-ops/docker-foo-bar:1` image would map
to `<local-registry-URI>/emulab-ops/emulab-ops/docker-foo-bar:1`; the
mapping is `<local-registry-URI>/pid/gid/imagename:version`.  Docker
repository names are lowercase-only, so we handle that for the user; but
I would prefer that users use lowercase Emulab imagenames for all Docker
images; that will help us.  That is not enforced in the code; it will
appear in the documentation, and we'll see.

Full Docker imaging relies on several other libraries
(https://gitlab.flux.utah.edu/emulab/pydockerauth,
https://gitlab.flux.utah.edu/emulab/docker-registry-py).  Each
Emulab-based cluster must currently run its own private registry to
support image loading/capture (note however that if capture is
unnecessary, users can use the external images path instead).  The
pydockerauth library is a JWT token server that runs out of boss's
Apache and implements authn/authz for the per-Emulab Docker registry
(probably running on ops, but could be anywhere) that stores images and
arbitrates upload/download access.  For instance, nodes in an experiment
securely pull images using their pid/eid eventkey; and the pydockerauth
emulab authz module knows what images the node is allowed to pull
(i.e. sched_reloads, the current image the node is running, etc).  Real
users can also pull images via user/pass, or bogus user/pass + Emulab
SSL cert.  GENI credential-based authn/z was way too much work, sadly.
There are other auth/z paths (i.e. for admins, temp tokens for secure
operations) as well.

As far as Docker image distribution in the federation, we use the same
model as for regular ndz images.  Remote images are pulled in to the
local cluster's Docker registry on-demand from their source cluster via
admin token auth (note that all clusters in the federation have
read-only access to the entire registries of any other cluster in the
federation, so they can pull images).  Emulab imageid handling is the
same as the existing ndz case.  For instance, image versions are lazily
imported, on-demand; local version numbers may not match the remote
image source cluster's version numbers.  This will potentially be a
bigger problem in the Docker universe; Docker users expect to be able to
reference any image version at any time anywhere.  But that is of course
handleable with some ex post facto synchronization flag day, at least
for the Docker images.

The big new thing supporting native Docker image usage is the guts of a
refactor of the utils/image* scripts into a new library, libimageops;
this is necessary to support Docker images, which are stored in their
own registry using their own custom protocols, so not amenable to our
file-based storage.  Note: the utils/image* scripts currently call out
to libimageops *only if* the image format is docker; all other images
continue on the old paths in utils/image*, which all still remain
intact, or minorly-changed to support libimageops.

libimageops->New is the factory-style mechanism to get a libimageops
that works for your image format or node type.  Once you have a
libimageops instance, you can invoke normal image logical operations
(CreateImage, ImageValidate, ImageRelease, et al).  I didn't do every
single operation (for instance, I haven't yet dealt with image_import
beyond essentially generalizing DownLoadImage by image format).
Finally, each libimageops is stateless; another design would have been
some statefulness for more complicated operations.   You will see that
CreateImage, for instance, is written in a helper-subclass style that
blurs some statefulness; however, it was the best match for the existing
body of code.  We can revisit that later if the current argument-passing
convention isn't loved.

There are a couple outstanding issues.  Part of the security model here
is that some utils/image* scripts are setuid, so direct libimageops
library calls are not possible from a non-setuid context for some
operations.  This is non-trivial to resolve, and might not be worthwhile
to resolve any time soon.  Also, some of the scripts write meaningful,
traditional content to stdout/stderr, and this creates a tension for
direct library calls that is not entirely resolved yet.  Not hard, just
only partly resolved.

Note that tbsetup/libimageops_ndz.pm.in is still incomplete; it needs
imagevalidate support.  Thus, I have not even featurized this yet; I
will get to that as I have cycles.