Water expands when it freezes so what happens if you completely fill a really strong container that can't expand with water and tried to freeze it in a freezer?

Water will freeze even under enormous pressure and expansion is not necessary for water to freeze. Depending on temperature and pressure, Ice can exist in 15 different phases (Ice I - Ice XV).

By compressing water for example at a temperature of -83 ºC (-117 ºF) and at 300 MPa, Ice II will form. Ice II is a rhombohedral crystalline form of ice. Your freezer is only cold enough for Ice I, so water would not turn to ice if you had a container strong enough to prevent expansion.