Issue #3673: start with a local composite action

.github/actions/docker_image_builder/action.yml

@@ -0,0 +1,29 @@
+%YAML 1.1
+---
+
+# Local composite action for building Docker images and pushing them to Docker Hub.
+# Intended to be uses by the workflows docker_image_builder_rel.yml and
+# docker_image_builder_devel.yml
+name: 'DockerImageBuilder'
+
+description: 'Composite action for building Docker images and pushing them to Docker Hub'
+
+inputs:
+  dummy-input:
+    description: 'only as a test'
+    required: true
+
+outputs:
+
+runs:
+
+  using: 'composite'
+
+  steps:
+
+      - name: 'devel output'
+        run: |
+          echo 'inputs.dummy-input: ${{ inputs.dummy-input }}'
+
+      - name: 'Set up Docker Buildx'
+        uses: docker/setup-buildx-action@v3

.github/workflows/docker_image_builder_devel.yml

@@ -96,9 +96,13 @@ jobs:
               - '${{ matrix.context }}/**'
 
       -
-        name: Set up Docker Buildx
+        # Uses a local composite action.
+        # The repository must already be checked out.
+        name: 'Run local composite action'
         if:   steps.changes.outputs.context == 'true'
-        uses: docker/setup-buildx-action@v3
+        uses: ./.github/actions/docker_image_builder
+        with:
+          dummy-input: 'just testing. devel'
 
       -
         # Build the image but do not push it to Docker Hub yet.

.github/workflows/docker_image_builder_rel.yml

@@ -81,3 +81,15 @@ jobs:
           echo 'github.ref_name: ${{ github.ref_name }}'
           echo 'github.ref_type: ${{ github.ref_type }}'
           echo 'github.event_name: ${{ github.event_name }}'
+
+      - name: 'check out the relevant OTOBO branch'
+        uses: actions/checkout@v4
+
+      -
+        # Uses a local composite action.
+        # The repository must already be checked out.
+        name: 'Run local composite action'
+        #if:   steps.changes.outputs.context == 'true'
+        uses: ./.github/actions/docker_image_builder
+        with:
+          dummy-input: 'just testing, rel'