This spack command adds a new schema for a file which describes the
builder containers available, along with the compilers availabe on
each builder. The release-jobs command then generates the .gitlab-ci.yml
file by first expanding the release spec set, concretizing each spec
(in an appropriate docker container if --this-machine-only argument is
not provided on command line), and then combining and staging all the
concrete specs as jobs to be run by gitlab.
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