Platform specific health checks #177
Description
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