As per this documentation, currently, it is not possible to mount volumes outside $BITBUCKET_CLONE_DIR.

Bitbucket Pipeline throws the following error if the build tries to mount a volume outside $BITBUCKET_CLONE_DIR. 

ERROR: failed to build: failed to create 'creator' container: Error response from daemon: authorization denied by plugin pipelines: -v only supports $BITBUCKET_CLONE_DIR and its subdirectories

This feature request is to allow mounting volumes outside $BITBUCKET_CLONE_DIR in Bitbucket pipelines.