Refactor GitHub Actions workflow for image building

.github/workflows/build.yml

@@ -1,8 +1,6 @@
 name: Build Container Images
 
 on:
-  push:
-    branches: [ "main" ]
   schedule:
     - cron: "0 4 * * *"
   workflow_dispatch:
@@ -12,8 +10,26 @@ env:
   IMAGE_PREFIX: ${{ github.repository }}
 
 jobs:
-  build:
+  discover:
     runs-on: ubuntu-latest
+    outputs:
+      images: ${{ steps.find.outputs.images }}
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Find image directories
+        id: find
+        run: |
+          images=$(find . -name Dockerfile -not -path './.github/*' \
+            | sed 's|/Dockerfile||; s|^\./||' \
+            | jq -R -s -c 'split("\n") | map(select(length > 0))')
+          echo "images=${images}" >> "$GITHUB_OUTPUT"
+
+  build:
+    needs: discover
+    runs-on: ubuntu-latest
+    if: ${{ needs.discover.outputs.images != '[]' }}
     permissions:
       contents: read
       packages: write
@@ -21,11 +37,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        image:
-          - name: rpmbuild
-            context: ./rpmbuild
-          - name: kernel
-            context: ./kernel
+        image: ${{ fromJson(needs.discover.outputs.images) }}
 
     steps:
       - name: Checkout repository
@@ -45,15 +57,15 @@ jobs:
         id: meta
         uses: docker/metadata-action@v5
         with:
-          images: ${{ env.REGISTRY }}/${{ env.IMAGE_PREFIX }}/${{ matrix.image.name }}
+          images: ${{ env.REGISTRY }}/${{ env.IMAGE_PREFIX }}/${{ matrix.image }}
           tags: |
             type=raw,value=latest
             type=raw,value={{date 'YYYYMMDD'}}
 
-      - name: Build and push ${{ matrix.image.name }}
+      - name: Build and push ${{ matrix.image }}
         uses: docker/build-push-action@v5
         with:
-          context: ${{ matrix.image.context }}
+          context: ./${{ matrix.image }}
           push: true
           tags: ${{ steps.meta.outputs.tags }}
           labels: ${{ steps.meta.outputs.labels }}