docker: checkpatch: Add missing python3-ply
checkpatch calls spdxcheck that depends on ply:
Traceback (most recent call last): File "/builds/linux-media/users/sebastianfricke/scripts/spdxcheck.py", line 6, in from ply import lex, yacc ModuleNotFoundError: No module named 'ply'
Signed-off-by: Ricardo Ribalda ribalda@chromium.org