Superbuild CI on Centos 6

Summary

Integration of Centos 6 platform using SuperBuild into CI system.

Rationale

Required for new CI system.

Implementation Details

Added centos-superbuild-base Dokerfile. Added jobs for Centos 6 in .gitlab-ci.yml.

Additional notes

NA

Copyright

The copyright owner is CNES and has signed the ORFEO ToolBox Contributor License Agreement.

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Check before merging:

• All discussions are resolved
• At least 2 votes from core developers, no vote.
• The feature branch is (reasonably) up-to-date with the base branch
• Dashboard is green
• Copyright owner has signed the ORFEO ToolBox Contributor License Agreement