docker: Fix cmake version to 3.31.6

Previously the newest available version of cmake was always installed in
the shrinkwrap docker image. But since v4.0.0 was released, we started
getting an error when compiling the optee client as part of buildroot
(our default buildroot config enables optee):

---8<---
CMake Error at CMakeLists.txt:1 (cmake_minimum_required):
  Compatibility with CMake < 3.5 has been removed from CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

  Or, add -DCMAKE_POLICY_VERSION_MINIMUM=3.5 to try configuring anyway.
---8<---

The issue is that optee doesn't explicitly specify a policy version so
cmake assumes a very old one which is no longer supported in v4.0.0.
Ultimately this needs to be fixed in buildroot's upstream, but until
then, let's fix the version at the newest v3 point release.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>