Vendor the `repro-build` script in our codebase, which will be used to
build our container image in a reproducible manner. We prefer to copy it
verbatim for the time-being, since its interface is not stable enough,
and the repro-build repo is not reviewed after all.
In the future, we want to store this script in a separate place, and
pull it when necessary.
Refs #1085
