always use the "real" latest version of ostc-builder image

in yoctobuild.dockerfile we had:

FROM registry.ostc-eu.org/ostc/containers/ostc-builder:latest

but in this way it didn't update the docker image when the ostc-builder image was updated.

I temporarily changed it to

FROM registry.ostc-eu.org/ostc/containers/ostc-builder@sha256:31e82698fc596f2986a3b9442fc0190563ef6ed65b0007a5b315b5c33c98f06e

in order to point to the current latest version, but this should be changed and done like we do for tinfoilhat