Fix ownership on artifact copies
The implementation of D984 did not include enforcing ownership for
build.CopyFrom instruction and so artifacts copied from one image to
another via copies: were problematically owned as root.
In order to fix this behavior:
- config.ArtifactConfig build.CopyFrom instructions are now injected duration build.PhaseInstall
- config.VariantConfig calls build.ApplyUser for these artifact instructions as well using the runs.as user
- build.CopyAs was refactored to wrap any build.Instruction which should only really be used with build.Copy or build.CopyFrom.
Run go test ./.... Run blubber against configuration with a variant that
uses copies and verify that the COPY --from instructions also include a
Reviewers: thcipriani, mmodell, hashar, Release-Engineering-Team, demon
Reviewed By: thcipriani, Release-Engineering-Team
Differential Revision: https://phabricator.wikimedia.org/D1002