Merge remote-tracking branch 'remotes/philmd/tags/gitlab-ci-20210527' into staging

Merge remote-tracking branch 'remotes/philmd/tags/gitlab-ci-20210527' into staging

GitLab CI patches queue

- Explode .gitlab-ci.yml in reusable templates
- Add job to cross build/test TCI on i386 host
- Remove CentOS 7 linux-user build job
- Temporarily set Avocado-based jobs in manual mode
- Increase time to hold Avocado reports to 1 week

# gpg: Signature made Thu 27 May 2021 15:40:58 BST
# gpg:                using RSA key FAABE75E12917221DCFD6BB2E3E32C2CDEADC0DE
# gpg: Good signature from "Philippe Mathieu-Daudé (F4BUG) <f4bug@amsat.org>" [full]
# Primary key fingerprint: FAAB E75E 1291 7221 DCFD  6BB2 E3E3 2C2C DEAD C0DE

* remotes/philmd/tags/gitlab-ci-20210527:
  gitlab: Convert check-dco/check-patch jobs to the 'rules' syntax
  gitlab: Use $CI_DEFAULT_BRANCH instead of hardcoded 'master'
  gitlab: Run Avocado tests manually (except mainstream CI)
  gitlab: Keep Avocado reports during 1 week
  gitlab: Extract cross-container jobs to container-cross.yml
  gitlab: Document how forks can use different set of jobs
  gitlab: Move current job set to qemu-project.yml
  gitlab: Extract all default build/test jobs to buildtest.yml
  gitlab: Drop linux user build job for CentOS 7
  gitlab: Extract core container jobs to container-core.yml
  gitlab: Extract default build/test jobs templates
  gitlab: Extract build stages to stages.yml
  gitlab: Extract DCO/style check jobs to static_checks.yml
  gitlab: Extract crossbuild job templates to crossbuild-template.yml
  gitlab: Extract container job template to container-template.yml
  gitlab: Enable cross-i386 builds of TCI
  gitlab: Rename ACCEL_CONFIGURE_OPTS to EXTRA_CONFIGURE_OPTS
  gitlab: Replace YAML anchors by extends (acceptance_test_job)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>