Fix Docker Hub tags list specification

The current implementation of the Docker Hub returns a list of objects
containing the tag name and the layer id.

```json
$ curl https://registry.hub.docker.com/v1/repositories/redis/tags
[{"layer": "ef16032a", "name": "latest"}, {"layer": "51345c84", "name": "2.6"}, {"layer": "51345c84", "name": "2.6.17"}, {"layer": "ef16032a", "name": "2.8"}, {"layer": "9554c310", "name": "2.8.10"}, {"layer": "fb968d8e", "name": "2.8.11"}, {"layer": "a2f8e595", "name": "2.8.12"}, {"layer": "1e665a02", "name": "2.8.13"}, {"layer": "9a809f46", "name": "2.8.14"}, {"layer": "b1985912", "name": "2.8.15"}, {"layer": "7c66279c", "name": "2.8.16"}, {"layer": "ef16032a", "name": "2.8.17"}, {"layer": "6064dc06", "name": "2.8.6"}, {"layer": "818c4318", "name": "2.8.7"}, {"layer": "acf48e3e", "name": "2.8.8"}, {"layer": "e938c5d0", "name": "2.8.9"}]
```