feat: use Cloud Native Buildpacks by default
BREAKING CHANGE: Auto Build Image now defaults to Cloud Native
Buildpacks instead of Herokuish when no Dockerfile is present. Users
depending on the /bin/herokuish binary provided by Herokuish should
either change their deployments to use /cnb/lifecycle/launcher instead
of /bin/herokuish exec with , or opt-out of using Cloud Native
Buildpacks, by setting the CI variable AUTO_BUILD_IMAGE_CNB_ENABLED to
false.
Edited by Hordur Freyr Yngvason