add a volume for faster init

[pj050806]

https://github.com/buschlab/MIRACUM-cbioportal/blob/92a442e7e82962cedea979b9fe7d6be51de0f177/docker-compose.yml#L218C1-L218C1

consider adding a volume

volumes: 
  - vol:/var/lib/mysql

for faster initialization and data persistence

[nr23730]

This image actually comes preloaded with data. So adding a volume to that mount point would override the data that is shipped with the container, and so break the functionality of the ensembl-rest container that uses this database.

[nr23730]

For details you can look up the files here: https://github.com/buschlab/MIRACUM-cbioportal/tree/master/services/ensembl-mysql

[pj050806]

According to my test and what is described here as node_modules volume trick and also here the data won't change.