Ok but how in the dock do I find the hash in "TAG=6.0.0-beta1-3eab5b40"

Okay so this is my .env right now. How do I find the respective hash (3eab5b40) for my desired version (7.0.0-beta1)?

TAG=7.0.0-beta1
ELASTIC_VERSION=7.0.0-beta1

# Also be sure to set the ELASTIC_VERSION variable. For released versions,
  # ${TAG} and ${ELASTIC_VERSION} will be identical, but for pre-release
  # versions, ${TAG} might contain an extra build identifier, like
  # "6.0.0-beta1-3eab5b40", so a full invocation might look like:
  #
  #   ELASTIC_VERSION=6.0.0-beta1 TAG=6.0.0-beta1-3eab5b40 docker-compose up
  #