[dock] Implement environment discover 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.
4 jobs for 77_implement_label_based_search in 19 minutes and 48 seconds (queued for 1 second)