Add support for listing container image labels

[exdx]

Use-case:
I want to be able to see the labels on the particular container image I am interested in, to inform what I would like to copy.

A/C:
Provide a dcp describe command that can show the labels associated with a particular image.

[exdx]

docker does have a docker image inspect subcommand that provides all metadata around the image, including the labels -- this command could leverage that to simply call that command, return the output, and exit

[exdx]

Or we can simply suggest to actually use docker image inspect on the image first, and not put this functionality into dcp, similar to #52