Great - this is almost exactly what I came up with independently before revisiting your instructions that I had saved months ago. However, the persistance of the express db (/opt/oracle/oradata) is apparently dependent on the host folder's permissions. The host folder must be owned by container user UID/GID 54321:

• stackoverflow.com/questions/47697474/persisting-data-in-dockers-volume-for-oracle-database
• stackoverflow.com/a/45673309/18961948