comparator for docker images

```
comparators:
docker:
- image: image/name # will look for first version found locally unless specified
script: cd foo && docker build # to build the docker image
```

Start by locating the existing comparator implementations and the configuration entry point for the shown comparators.docker structure. Determine how an image and optional build script should be handled, including the stated local-version lookup, and verify the completed comparator through the repository's existing checks.

Written by the indexing model from the issue text.