Platform specific health checks

[tmcgilchrist]

OBuilder has support for running health checks to identify issues with the platform it is running on. Presently those checks assume a Linux platform with Docker being available. eg

$ obuilder healthcheck --store=zfs:tank
Healthcheck passed

Now that OBuilder has support for FreeBSD and macOS that do not use docker, we should drop the docker installation requirement and implement a more suitable health check on those platforms.

In particular on FreeBSD, we install a docker client binary to satisfy this requirement but never use it to perform builds. Similarly for macOS