ci: add action to determine next image revision

.github/actions/get-image-revision/action.yml

@@ -0,0 +1,78 @@
+name: "Get Incremented Revision Tag"
+description: "For a pre-existing rolling tag lookup the latest immutable revision tag, increment and return"
+inputs:
+  image-ref:
+    description: "The full image reference including namespace, repo and tag"
+    required: true
+  revision-separator:
+    description: "The separator string to use between tag and revision number. Defaults to '-r'"
+    required: false
+    default: "-r"
+  tag-options-prefix:
+    description: "A string that the revision tag will be appended to and sent to output tag-options"
+    required: false
+    default: ""
+outputs:
+  revision-tag:
+    description: "The updated immutable tag, ready to use"
+    value: ${{ steps.extract.outputs.revision_tag }}
+  tag-options:
+    description: "A string with some command line options (may be empty)"
+    value: ${{ steps.extract.outputs.tag_options }}
+runs:
+  using: composite
+  steps:
+    - shell: bash
+      id: extract
+      run: |
+        IMAGE="${{ inputs.image-ref }}"
+        IMAGE_NS_REPO="${IMAGE%:*}"
+        IMAGE_TAG="${IMAGE#*:}"
+
+        if [[ "$IMAGE_TAG" = "$IMAGE_NS_REPO" ]]; then
+          >&2 echo "You must provide an image reference in the format [<namespace>/]<repo>:<tag>"
+          exit 1
+        fi
+
+        case "$IMAGE_NS_REPO" in
+            */*) :;; # namespace/repository syntax, leave as is
+            *) IMAGE_NS_REPO="library/$IMAGE_NS_REPO";; # bare repository name (docker official image); must convert to namespace/repository syntax
+        esac
+
+        # Without such a token we run into rate limits
+        token=$( curl -s "https://auth.docker.io/token?service=registry.docker.io&scope=repository:$IMAGE_NS_REPO:pull" )
+
+        ALL_TAGS="$(
+          i=0
+          while [ $? == 0 ]; do
+              i=$((i+1))
+              RESULT=$( curl -s -H "Authorization: Bearer $token" "https://registry.hub.docker.com/v2/repositories/$IMAGE_NS_REPO/tags/?page=$i&page_size=100" )
+              if [[ $( echo "$RESULT" | jq '.message' ) != "null" ]]; then
+                # If we run into an error on the first attempt, that means we have a problem.
+                if [[ "$i" == "1" ]]; then
+                  >&2 echo "Error when retrieving tag data: $( echo "$RESULT" | jq '.message' )"
+                  exit 2
+                # Otherwise it will just mean we reached the last page already
+                else
+                  break
+                fi
+              else
+                echo "$RESULT" | jq -r '."results"[]["name"]'
+                # DEBUG:
+                #echo "$RESULT" | >&2 jq -r '."results"[]["name"]'
+              fi
+          done
+        )"
+
+        # Note: if a former tag could not be found, it just might not exist already. Start new series with rev 0
+        CURRENT=$( echo "$ALL_TAGS" | grep "${IMAGE_TAG}${{ inputs.revision-separator }}" | sed -e "s#${IMAGE_TAG}${{ inputs.revision-separator }}##" | sort -h | tail -n1 )
+        if [[ "$CURRENT" ]]; then
+          REVISION_TAG="${IMAGE_TAG}${{ inputs.revision-separator }}$((CURRENT+1))"
+        else
+          REVISION_TAG="${IMAGE_TAG}${{ inputs.revision-separator }}0" 
+        fi
+        echo "revision_tag=${REVISION_TAG}" | tee -a "$GITHUB_OUTPUT"
+        
+        if [[ -n "${{ inputs.tag-options-prefix }}" ]]; then
+          echo "tag_options=${{ inputs.tag-options-prefix }}${REVISION_TAG}" | tee -a "$GITHUB_OUTPUT" 
+        fi