Conform to the API spec and stop returning image URIs in the /v2/_catalog route

[dalazx]

Currently our catalog route returns image URIs which is inconsistent to the API spec:

{                                                           
  "repositories": [                                   
    "image://adavydow-ssh",                    
    "image://adavydow/adavydow-ssh",                        
    "image://adavydow/artem_sftp_client",     
    "image://adavydow/artem_ssh",  

see https://docs.docker.com/registry/spec/api/#listing-repositories