Add a CI pipeline

[MeltyBot]

Migrated from GitLab: https://gitlab.com/meltano/files-airflow/-/issues/6

Originally created by @DouweM on 2021-04-01 21:48:07

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I think we can add a meltano.yml to this project that comes with:

1. orchestrator airflow
2. a file bundle with pip_url: . (so that it always uses the most recent version from the current branch)
3. some schedules

Then we can add a .gitlab-ci.yml that runs meltano invoke airflow scheduler and validates the expected output, i.e. there should be no error about meltano.py, or the expected lines confirming that a DAG was created for each schedule.

Similar to how https://gitlab.com/DouweM/tap-investing tests itself using https://gitlab.com/DouweM/tap-investing/-/blob/master/meltano.yml and https://gitlab.com/DouweM/tap-investing/-/blob/master/.gitlab-ci.yml#L18.