Remove anonymous volumes by default
After this change Dockertest removes anonymous volumes by default. Containers built from the Dockerfile that defines anonymous volumes via VOLUME
directive will stop leaking storage.
Named volumes aren't affected by the change.
New prune strategy "always_keep_anonymous_volumes" is introduced to mimic current behaviour and could be used for cases when anonymous volumes are shared between containers and cleaned explicitly.
Edited by Rinat Shigapov