Improve grouping of sync containers

[grayside]

Sync containers currently appear in the unclassified section of the dashboard. This is a bit misleading, as these containers are usually critical pieces of the plumbing of specific projects.

But what can we do to improve this? Two options occur to me.

1. In our most common usage, the sync container is generally "universal" to every locally running instance of the project. Therefore show the sync container in each grouping of containers. (Currently there could be multiple groupings for a single project, by virtue of different docker-compose project label.)
2. Create a category of sync containers. They have some pretty special characteristics anyway and are more likely to be subject to similar oversight and management.

In either case, we may want to modify rig to add a sync container label.