Watchers should more easily work with generic containers

[stellarmagnet]

We want to move towards watchers working with a generic container that can either accept contracts or watch head, and have a consistent way for how they are configured, such that we can implement a continuous deployment process.

(Right now, if we want the watcher to watch a specific contract, it is not very clear how to pass parameters for configs or what those contracts are to a generic container. E.g. The lighthouse watcher is running under system d, and it's not containerized.)