KandraMultiDcFixtureException

Thrown by KandraMultiDcTestcontainers when the multi-DC Docker Compose fixture itself fails to come up -- a dynamically-chosen port still collided (see KandraMultiDcTestcontainers.compose's doc for the small, inherent check-then-bind race), Docker isn't running, or Compose otherwise failed to start. Wraps whatever Testcontainers/Docker raised with an actionable, Kandra-authored message instead of leaving a caller to decode a raw Docker bind error (GH #108 / ISS-095).