Docker-compose and Unknown MySQL server host

In case you encounter this error with mysql and your docker-compose setup:

Unknown MySQL server host <mysql-service-name>

… and you dont know why because everything seems to be correct.

Then you might have an upgrade problem with mysql because you are reusing an old volume that was created for another mysql version.
This can happen when you use a unspecified tag as mysql:latest (not recommended anyway) and there was a version bump in the official mysql image.
Or you upgraded the mysql container yourself, f.e. from mysql:5.6 to mysql:5.7 and you are reusing the data volume with the mysql files.

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