Improvement to 'containerize.sh' features after Buildah version updated

[aperloff]

This issue is related to files which will be added with #18.

The version of Buildah on cmslpcheavy-sl7 right now is 1.11.6, but the latest version is 1.21.0. The later versions have a feature that allow the use of ARG commands within a COPY commands parameters (i.e. chown) inside of a Dockerfile. This would allow us to remove the hard coded cmsusr username in https://github.com/FNALLPC/lpc-scripts/pull/18/files#diff-64e49c956c3cc82ad01ae4cc0497f5ee63343fa4b61c8c7733199205ca5bd187R47. That line would change to be something like:

ARG USERNAME_BUILD=cmsusr
ARG USERNAME=cmsusr
COPY --from=builder --chown=${USERNAME}:${USERNAME} /home/${USERNAME_BUILD}/${CMSSW_VERSION} /home/${USERNAME}/${CMSSW_VERSION}