Docker Container Volumes Synced Locally

I am trying to run a local Docker Registry with a synchronized storage with my Dropbox so I can have access to built VMs without having to rebuild the images if I move to a different work station.

I have launched the Registry with the following:

docker run -d -p 5000:5000 --name td-registry -v /Users/andrew/Dropbox/Developer/Docker/Registry:/tmp/registry registry

Then, I push an example image that I have built via:

docker push localhost:5000/ubunutu

Now, I would expect to see some sort of file or directory placed into /Users/andrew/Dropbox/Developer/Docker/Registry; but that is not the case:

$ ls -lah ~/Dropbox/Developer/Docker/Registry
total 0
drwxr-xr-x  2 andrew  staff    68B Sep 24 14:39 .
drwxr-xr-x  4 andrew  staff   136B Sep 24 16:42 ..

Am I mounting the data volume incorrectly when launching the Registry container?

Edit

Here is the output of the docker push, so it's successfully pushing.

The push refers to a repository [localhost:5000/ubuntu] (len: 1)
Sending image list
Pushing repository localhost:5000/ubuntu (1 tags)
511136ea3c5a: Image successfully pushed
bfb8b5a2ad34: Image successfully pushed
c1f3bdbd8355: Image successfully pushed
897578f527ae: Image successfully pushed
9387bcc9826e: Image successfully pushed
809ed259f845: Image successfully pushed
96864a7d2df3: Image successfully pushed
a037e7415015: Image successfully pushed
922d395cc25c: Image successfully pushed
f9317ffe9a11: Image successfully pushed
1a980360e853: Image successfully pushed
f759631e9b64: Image successfully pushed
194edb5b619b: Image successfully pushed
5cf96e6ae328: Image successfully pushed
b4d4b1e2e0b3: Image successfully pushed
921507f17768: Image successfully pushed
b9faffd3f579: Image successfully pushed
Pushing tag for rev [b9faffd3f579] on {http://localhost:5000/v1/repositories/ubuntu/tags/latest}