doc/CMake: Add a note for using Docker in CMake readme

Docker releases are unstable so using it in SCP build
could result in some errors. A note is added to highlight
this problem.

Signed-off-by: Ahmed Gadallah <ahmed.gadallah@arm.com>
Change-Id: Ia51efd1af36d75fe8f3a103f4006e615dec663e1