Merge pull request #1215 from Frederik-Baetens/main

update sortable image tag guide with github.run_number

docs/guides/sortable-image-tags.md

@@ -36,6 +36,7 @@ $ date +%s
 Alternatively, you can use a serial number as part of the tag.  Some CI platforms will provide a
 build number in an environment variable, but that may not be reliable to use as a serial number --
 check the platform documentation.
+For example, Github makes availabe the variable `github.run_number` which can be used as a reliable ever increasing serial number.
 
 A commit count can be a reasonable stand-in for a serial number, if you build an image per commit
 and you don't rewrite the branch in question:
@@ -105,6 +106,39 @@ jobs:
             ${{ env.IMAGE }}:${{ steps.prep.outputs.BUILD_ID }}
 ```
 
+### Alternative example utilizing github.run_number
+
+Here is another example example of a [GitHub Actions job][gha-syntax] which tags images using Github action's built in `run_number`
+and the git SHA1:
+
+```yaml
+jobs:
+  build-push:
+    env:
+        IMAGE: org/my-app
+    runs-on: ubuntu-latest
+    steps:
+    # These are prerequisites for the docker build step
+    - name: Set up QEMU
+      uses: docker/setup-qemu-action@v1
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v1
+    - name: Login to DockerHub
+      uses: docker/login-action@v1
+      with:
+          username: ${{ secrets.DOCKERHUB_USERNAME }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
+
+    - name: Build and publish container image with tag
+      uses: docker/build-push-action@v2
+      with:
+          push: true
+          context: .
+          file: ./Dockerfile
+          tags: |
+            ${{ env.IMAGE }}:${{ github.sha }}-${{ github.run_number }}
+```
+
 ## Using in an `ImagePolicy` object
 
 When creating an `ImagePolicy` object, you will need to extract just the timestamp part of the tag,
@@ -112,7 +146,7 @@ using the `tagFilter` field. You can filter for a particular branch to restrict
 built from that branch.
 
 Here is an example that filters for only images built from `main` branch, and selects the most
-recent according the timestamp (created with `date +%s`):
+recent according to a timestamp (created with `date +%s`) or according to the run number (`github.run_number` for example):
 
 ```yaml
 apiVersion: image.toolkit.fluxcd.io/v1alpha2
@@ -124,6 +158,7 @@ spec:
   imageRepositoryRef:
     name: image-repo
   filterTags:
+    ## use "pattern: '(?P<ts>.*)-.+'" if you copied the workflow example using github.run_number
     pattern: '^main-[a-f0-9]+-(?P<ts>[0-9]+)'
     extract: '$ts'
   policy: