Follow up to #2434, this lets Renovate actually update those image lines

Add support for renovating Docker images in Kubernetes manifests

Removes custom Docker lookup code and instead integrates it with the generic lookup routine used by other package managers. Logic for digest support was added but is used by Docker-only for now.

Closes #2081, Closes #2276

Adds support for renovating Docker Compose files (e.g. `docker-compose.yml`). Functionality is essentially the same as the existing `Dockerfile` capabilities, so config for `docker` is shared with `docker-compose` but may also be overridden.

Merging as disabled by default - will wait for some opt-in testing before turning it on by default.

Closes #832