Fix some problems with Docker container

[oliver]

This PR fixes two problems I encountered when setting up the Docker container for MySQL database:

• at container startup there was an error message: /package/admin/s6-overlay-3.2.0.2/etc/s6-rc/scripts/cont-init: line 20: /etc/cont-init.d/95_mediathekview_db: Permission denied.
  To fix this, the docker/95_mediathekview_db is now marked as executable in Git.
• when the mvupdate3 tool was running there was a Python error: TypeError: MySQLCursor.execute() got an unexpected keyword argument 'multi'. It looks like beginning with mysql-connector-python 9.2.0 the cursor.execute() method does not support the multi parameter any more (see docs and changelog).
  I did not quite understand how to migrate to the suggested replacement feature (Executing Multiple Statements) – maybe fetchall() needs to be used now? To fix this for now I have explicitly installed mysql-connector-python version 9.1.0.

Also, I noticed that when running the container with an empty data directory and with RUN_ON_STARTUP=yes, mvupdate3 would be started while MySQL was still starting up; and then mvupdate3 would fail with error message mysql.connector.errors.InterfaceError: 2003: Can't connect to MySQL server on 'localhost:3306' (Errno 111: Connection refused). Restarting the container fixed this (because then the database already existed).

Thank you for providing this plugin!