Use kaoto-app image digest when publish the operator

[lburgazzoli]

As today, the operator default to the quay.io/kaotoio/kaoto-app:stable image which is fine but to enforce kaoto's web app re-deployment and update when the operator gets updated, we must reference the kaoto-app image digest.

i.e. we can set the KAOTO_APP_IMAGE env var before building the operator in the build-and-push workflow:

export KAOTO_APP_IMAGE=$(crane digest quay.io/kaotoio/kaoto-app:stable)
make docker-build