Stack with 'getMinimum' operation
You have a bug: .getMinimum() loses track if you push the same new minimum value twice.

You should use both inheritance and composition.

Use inheritance for the "main" stack, because your data structure is a stack — one with an extra feature.  That gives you the read-only operations .peek(), .size(), and .isEmpty() for free.

Use composition for the "minimum" stack, as you have currently done.