Describe Contribution Guide

docker/coturn/CONTRIBUTING.md

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+Contribution Guide
+==================
+
+
+
+
+## Best practices
+
+1. Keep the image size as small as possible:
+    - Do not produce redundant layers in the final image.
+    - Cleanup temporary files and caches in the same layer were they were produced.
+    - Remove unnecessary man pages, examples and documentation.
+
+2. Build each project in its separate stage:
+    - Do use layers granularly in non-final stages for better caching of build results.
+    - Prepare all the final files as much as possible in their build stage before adding them to the final stage.
+
+
+
+
+## CI workflow
+
+At the moment `coturn/coturn` Docker image's [workflow is automated][1] via [GitHub Actions] in the following manner:
+
+- On each push the image is built and tested.  
+  This helps to track image regressions due to changes in codebase.
+
+- Image is built and tested automatically from `master` branch on weekly basis.  
+  This helps to track image regressions due to changes in parent OS images (`debian`, `alpine`), their system packages, and other dependencies.
+
+- On each push to `master` branch the image is published with `edge-debian` and `edge-alpine` tags.  
+  This helps to test and try the latest `master` branch and its changes for whoever needs this.
+
+- On each `docker/X.Y.Z-rN` tag creation the image is built using the `X.Y.Z` Coturn version (not the local sources), tested, and is published with all the version tags declared in [`Makefile`] (see `ALL_IMAGES`).  
+  An appropriate [GitHub Release] for the `docker/X.Y.Z-rN` Git tag is also created automatically.
+
+- Whenever the image is published, its description on container registries is automatically updated with its [README] file.
+
+
+
+
+## Releasing
+
+To produce a new release (version tag) of `coturn/coturn` Docker image, perform the following steps:
+
+1. Upgrade the image version correctly in [`Makefile`] by bumping up either the `COTURN_VER` (if Coturn has changed it version) or the `BUILD_REV` (if anything else in the image has been changed). If the `COTURN_VER` has changed, the `BUILD_REV` may be reset to `0`.
+
+2. Complete an existing [CHANGELOG] or fill up a new one for the new version declared in [`Makefile`].
+
+3. Update [README] with the new version declared in [`Makefile`].
+
+4. Perform a `make release` command inside the`docker/coturn/` directory.
+
+
+
+
+
+[CHANGELOG]: https://github.com/coturn/coturn/blob/master/docker/coturn/CHANGELOG.md
+[GitHub Actions]: https://docs.github.com/actions
+[GitHub Release]: https://github.com/coturn/coturn/releases
+[README]: https://github.com/coturn/coturn/blob/master/docker/coturn/README.md
+
+[`Makefile`]: https://github.com/coturn/coturn/blob/master/docker/coturn/Makefile
+
+[1]: https://github.com/coturn/coturn/blob/master/.github/workflows/docker.yml