GitLab CI yaml includes custom tags that cannot be resolved by from_yaml

[haboustak]

GitLab has added a custom yaml tag: !reference. This tag copies content from another path within the yaml document. The example use cases are to reuse script blocks in other CI jobs. Implementation in ruby is here: lib/gitlab/ci/config/yaml/tags/reference.rb

I've created an example project based on the above documentation.

It seems appropriate to create a !from_gitlab custom tag to load GitLab-specific yaml syntax. This would use a SafeLoader with support for !reference initially. It would also provide a place for scuba to support some or all of GitLab's include syntax.

Sample .gitlab-ci.yml

stages:
  - build

.before:
  script:
    - echo "ABCD"

build:
  stage: build
  script:
    - !reference [.before, script]
    - echo "after ABCD completes"

[haboustak]

Two other GitLab-flavored yaml extensions from my recent memory that would be nice to have some level of some scuba support:

1. They build compound yaml documents using the includes keyword
2. They have replaced yaml anchors with the extends keyword

These extensions may be unreasonable to support within GitlabYamlLoader, but a warning from scuba would be nice. For example:

WARN: GitLabYamlLoader: Loading content from .gitlab-ci.yml with unsupported includes keyword.

[JonathonReinhart]

I think this all sounds great. I'm all for a !from_gitlab (!from_gitlab_ci_yaml or !from_gitlab_ci might be a bit too long).