Implement environment discovery using docker labels
The next release of BEAT environments will make use of docker labels to provide the information the current describe command returns (minus the os entry which is not currently used).
This new implementation of discovery uses these labels and thus speeds up the process.
The original code is kept as a fallback method in case a node only has images using the old build process.
The docker images concerned can be viewed in the beat/beat.env.builder> repo.
Part of #77