Embed BuildKit inline cache metadata in built images by default

[jasoncheng-jora]

• Images built by this plugin are meant to be reused as cache sources via --cache-from (e.g. by docker-ecr-publish), but BuildKit's default docker driver can only read inline cache metadata, which the plugin never embedded.
• Downstream builds pulled the cache image and then re-ran every layer anyway.
• Changes made:
  • Add --build-arg BUILDKIT_INLINE_CACHE=1 to the image build by default
  • Add a disable-cache-metadata: true opt-out, matching the option name in docker-ecr-publish
  • Document the behaviour in the README, including the additional-build-args: '--build-arg BUILDKIT_INLINE_CACHE=1' workaround for older versions