document image creation automation

Author: bmuenzenmeyer

CONTRIBUTING.md

@@ -10,6 +10,15 @@ New **NPM** releases are not tracked. We simply use the NPM version bundled in t
 
 **Yarn** is updated to the latest version only when there is a new Node.js SemVer PATCH release (unless Yarn has received a security update), and it's updated only in the branch with the new release, preferably in the same PR. The `update.sh` script does this automatically when invoked with a specific branch, e.g. `./update.sh 6.10`.
 
+### Image Creation Automation
+
+- Every 15 minutes, the [workflow](https://github.com/nodejs/docker-node/blob/main/.github/workflows/automatic-updates.yml) within the [nodejs/docker-node](https://github.com/nodejs/docker-node) repo [checks](https://github.com/nodejs/docker-node/blob/main/build-automation.mjs) for new versions of Node.js [published to the website's `index.json` file](https://nodejs.org/download/release/index.json).
+  - If found, it also checks for an [unofficial musl/Alpline build](https://unofficial-builds.nodejs.org/download/release/index.json).
+  - If found, the [update script](https://github.com/nodejs/docker-node/blob/main/update.sh) runs
+  - The workflow opens a pull request is opened either by [nodejs-github-bot](https://github.com/nodejs-github-bot). In some cases, this PR is manually created, such as new major releases.
+- Another [workflow](https://github.com/nodejs/docker-node/blob/main/.github/workflows/official-pr.yml) detects the merger of these pull requests and opens a pull request to [docker-library/official-images](https://github.com/docker-library/official-images).
+- The official images are built and published according to [docker's process](https://github.com/docker-library/faq#an-images-source-changed-in-git-now-what), resulting in the new images being available on [Docker Hub](https://hub.docker.com/_/node).
+
 ### Submitting a PR for a version update
 
 If you'd like to help us by submitting a PR for a version update, please do the following: