Hi JitteryCoyote63 ,
Anything related to the /var/lib/docker is actually out-of-scope for the Trains Server - we just use docker and docker-compose and cleaning up after docker is something that I'm sure can be further researched (we'll also appreciate any input on the subject) - I assume storage is mostly related to cached containers and console output being stored by docker-compose.
As for the ElasticSearch data (which is the main culprit from your list) - this is simply experiments data and trains agents reports being stored, and some of data can easily be cleared. Note that 30GB isn't that big, and that your experiments generate quite a bit of data. To see a breakdown of the data stored, you can simply query the ES for the list of all indices, sorted by storage size using http://<trains-hostname-or-ip>:9200/_cat/indices?v=true&s=store.size (of course, that's assuming port 9200 is open for external access. If not, just do it from the machine using curl http://localhost:9200/_cat/indices?v=true&s=store.size )