Too many layers in OSG docker containers

[DrDaveD]

Every RUN or COPY line in a Dockerfile ends up creating another docker layer. For apptainer that isn't a very big deal because it collapses all the layers when it downloads from dockerhub, but for docker and podman that requires another overlayfs level which will affect runtime performance. I think the OSG docker containers should be converted to use a single RUN command, so the only layers are from inherited containers.