Add option to group docker-containers by category

### Description

Currently it's possible to add multiple `docker-containers` widgets filtered on different category attributes. One issue is that it does not seem to be possible to only show containers that does not have a category.

I think it would by nice if the `docker-containers` widget had an option to group the containers by category.

The containers would be visually grouped together. The category string could be used as the header fore each group.

Any container that does not have a category could be grouped together at the end.

```yaml
- type: docker-containers
group-by-category: true
```

Start by locating the docker-containers widget implementation and the YAML option handling. Add the group-by-category behavior so categorized containers appear under category headers and uncategorized containers are grouped at the end; verify the widget still supports the existing category filters.

Written by the indexing model from the issue text.