docker-entrypoint.sh categorically overwrites numberOfTaskSlots

The docker entrypoint always runs this code:

TASK_MANAGER_NUMBER_OF_TASK_SLOTS=${TASK_MANAGER_NUMBER_OF_TASK_SLOTS:-1}
set_config_option taskmanager.numberOfTaskSlots ${TASK_MANAGER_NUMBER_OF_TASK_SLOTS} 

This always overrides the number of task slots, either with the value that was set by the user or otherwise, 1.

This means that users cannot mount a different config to change the number of slots.

We should only configure the number of slots if the environment variable was actually set.