Introducing GitHub Actions CI

Signed-off-by: Gianluca Barbon <gianluca.barbon@eurotech.com>

.github/workflows/README.md

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+# GitHub Actions CI Integration
+
+GitHub Actions allow building Continuous Integration pipeline for testing purposes.
+GitHub Actions are visible through the `Actions` tab in a GitHub repository, are triggered automatically for each push or pull-request and run also in forks.
+For further information about GitHub Actions, see the [official documentation](https://docs.github.com/en/free-pro-team@latest/actions).
+
+## Caching
+
+The GitHub Action [Cache](https://github.com/marketplace/actions/cache) allows reusing files between jobs and workflows.
+The content of the cache cannot be cleaned on purpose, but is cleaned when the hash of the files used for the key changes.
+This action is used in order to cache the maven dependencies.
+
+# Composite Steps
+
+Unfortunately, at the moment it is not possible to reuse some steps on multiple jobs.
+This feature is a nice to have, since it would be possible to reuse the steps that are common to all the test jobs.
+The feature has been already requested by the community, and it's being discussed here:
+- https://github.community/t/reusing-sharing-inheriting-steps-between-jobs-declarations/16851
+- https://github.com/actions/runner/issues/438
+
+The GitHub Actions dev team has already introduced the composite run steps, see:
+- https://github.com/actions/runner/blob/main/docs/adrs/0549-composite-run-steps.md
+- https://docs.github.com/en/free-pro-team@latest/actions/creating-actions/creating-a-composite-run-steps-action
+
+However, composite 'actions' are not supported yet, see:
+- https://github.com/actions/runner/issues/646
+- https://github.com/actions/runner/issues/646#issuecomment-734689627
+
+# Re-run single jobs
+
+It is currently not possible to restart single jobs.
+For more information see the following issues on GitHub Actions Community that are tracking this feature request:
+- https://github.community/t/ability-to-rerun-just-a-single-job-in-a-workflow/17234
+- https://github.community/t/re-run-jobs/16145/
+
+To cope with this missing feature, the [Retry](https://github.com/marketplace/actions/retry-step) GitHub Action is used.
+This action allows retrying a failed step for a fixed amount of attempts.