misc: add branch pipeline.

[grignards]

See #4886.

Checkpoints

For developers

• If relevant, manual tests have been done to ensure the stability of the whole application and that the involved feature works
• The original issue is up to date w.r.t the latest discussions and contains a short summary of the implemented solution
• Automated tests covering the feature or the fix, have been written, deemed irrelevant (give the reason), or an issue has been created to implement the test (give the link)
• Make sure that:
  • if there are modifications in the Tracim configuration files (eg. development.ini), they are documented in backend/doc/setting.md
  • any migration process required for existing instances is documented
  • relevant people for these changes are notified
• Original authors of the features included in a multi-feature branch (maintenance fixes -> develop, security fixes -> develop, …) should be part of the reviewers, especially if you encountered merge conflicts.

For code reviewers

• The code is clear enough
• If there are FIXMEs in the code, related issues are mentioned in the FIXME
• If there are TODOs, NOTEs or HACKs in code, the date and the developer initials are present

For testers

• Manual, quality tests have been done

[PhilippeAccorsi]

@grignards this PR is to improve concourse-ci ? If yes, I think we can merge without reviewers because we need to improve the ci quickly. if the four commits not improve ci for the moment, I understund to not merge this now.