Rephrase and try to be clearer regarding perl:onbuild
Rephrase and try to be clearer regarding perl:onbuild

Try to distinguish making the image for `perl:onbuild` and using as the
base image for the project.

Lightstreamer/docs,ceejatec/docs,zendtech/docs,docker-library/docs,docker-flink/docs,zendtech/docs,tilosp-docker/library-docs,davidcurrie/docs,docker-library/docs,tilosp-docker/library-docs,avoinea/docs,infosiftr/docker-library-docs,docker-solr/docs,davidcurrie/docs,31z4/docs,odoo/docker-docs,nats-io/docker-docs,docker-solr/docs,jetty-project/docker-docs,dmetzler/docker-library-docs,Bonitasoft-Community/docker_docs,dinogun/docs,docker-flink/docs,Bonitasoft-Community/docker_docs,avoinea/docs,odoo/docker-docs,31z4/docs,ceejatec/docs,jetty-project/docker-docs,Lightstreamer/docs,dmetzler/docker-library-docs,dinogun/docs,infosiftr/docker-library-docs,crate/docker-docs,crate/docker-docs,nats-io/docker-docs