Add full_image_build config option
When full_image_build is set to true, the container image build scripts will be called with FORCE_FULL_BUILD set to "true" in the environment.
Example use:
scap sync-world -Dfull_image_build:True testing
This setting will be used by https://gerrit.wikimedia.org/r/c/operations/puppet/+/869333
Bug: T325576