Very nice solution and good for learning!

It seems the tests do not test all requirements.

For example:
apples: 9 capacity: 20 max_left: 2

Should yield 7 (according to tests)

e.g.

buckets: 1 apples per bucket: 9 rest: 0
buckets: 2 apples per bucket: 4 rest: 1
buckets: 3 apples per bucket: 3 rest: 0
buckets: 4 apples per bucket: 2 rest: 1
buckets: 7 apples per bucket: 1 rest: 2
buckets: 8 apples per bucket: 1 rest: 1
buckets: 9 apples per bucket: 1 rest: 0

However this fails the criteria:
"if each box contains x apples, the number of left out apples should be less than x."

for:
buckets: 7 apples per bucket: 1 rest: 2 ( 2 > 1 )

oprypin inspired, that solution voted for Best Practises +1
Only needs to left strip a leading '-' since we won't have a trailing '-'