ci: add workflow to build ci img

Author: spwoodcock

.github/workflows/build_ci_img.yml

@@ -0,0 +1,33 @@
+name: 🔧 Build CI Img
+
+on:
+  push:
+    branches:
+      - main
+    paths:
+      # Workflow is triggered only if deps change
+      - "pyproject.toml"
+      - "Dockerfile"
+  # Allow manual trigger
+  workflow_dispatch:
+
+jobs:
+  backend-ci-build:
+    uses: hotosm/gh-workflows/.github/workflows/[email protected]
+    with:
+      build_target: ci
+      image_tags: |
+        "ghcr.io/${{ github.repository }}:ci"
+
+  invalidate-cache:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Delete CI Img Cache
+        env:
+          GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        run: |
+          gh extension install actions/gh-actions-cache
+          gh actions-cache delete image-cache-${{ runner.os }} \
+            -R ${{ github.repository }} \
+            -B ${{ github.ref_name }} \
+            --confirm || true