zephyr/docker-build.sh: upgrade CMake to 3.21 or above

CMake 3.21 changed the order object files are passed the linker. This
breaks build reproducibility.
Signed-off-by: Marc Herbert <marc.herbert@intel.com>

Author: marc-hb

zephyr/docker-build.sh

@@ -36,6 +36,12 @@ unset ZEPHYR_SDK_INSTALL_DIR
 ls -ld /opt/toolchains/zephyr-sdk-*
 ln -s /opt/toolchains/zephyr-sdk-*~/ ||true
 
+# CMake v3.21 changed the order object files are passed to the linker.
+# This makes builds before that version not reproducible.
+# To save time don't install if recent enough.
+pip install 'cmake>=3.21'
+PATH="$HOME"/.local/bin:"$PATH"
+
 iftest -e .west ||test -e zephyr;then
  init_update=''
 else