Add caching for tag lookup
Add caching for tag lookup

Adding a dedicated cache for tag lookup calls to the Docker Registry API.

rupakg/imagelayers,patocox/imagelayers,rupakg/imagelayers,CenturyLinkLabs/imagelayers,patocox/imagelayers,Acidburn0zzz/imagelayers,Acidburn0zzz/imagelayers,BWITS/imagelayers,BWITS/imagelayers,CenturyLinkLabs/imagelayers,argvader/imagelayers,argvader/imagelayers