ci: Add basic tests for built kas and kas-isar containers

Break up the build into two stages, performing basic tests with poky and
isar on the two kas container types before uploading them. The tests
consist of building zlib-native with poky and cowsay for x86 bullseye
with Isar.

Note that the ordering "build kas", "test kas", "push kas",
"build kas-isar" is important because "build kas-isar" will not use the
locally built and imported kas:next image and rather pull the previous
one from the registry.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

.github/workflows/next.yml

@@ -61,8 +61,8 @@ jobs:
           scripts/checkcode.sh .
           TERM=xterm pytest
 
-  deploy_containers:
-    name: Build and deploy container images
+  build_containers:
+    name: Build, test and deploy container images
     needs: perform_tests
     runs-on: ubuntu-latest
     steps:
@@ -81,14 +81,34 @@ jobs:
       - name: Build kas image
         uses: docker/build-push-action@v2
         with:
-          platforms: linux/amd64,linux/arm64
-          push: true
+          load: true
           tags: ghcr.io/siemens/kas/kas:next
+      - name: Test kas image
+        run: |
+          cd image-tests/poky
+          KAS_IMAGE_VERSION=next ../../kas-container build kas.yml
+      - name: Complete build and deploy kas image
+        uses: docker/build-push-action@v2
+        with:
+          platforms: linux/amd64,linux/arm64
+          tags: ghcr.io/siemens/kas/kas:next
+          push: true
       - name: Build kas-isar image
+        uses: docker/build-push-action@v2
+        with:
+          file: Dockerfile.isar
+          load: true
+          build-args: KAS_TAG=next
+          tags: ghcr.io/siemens/kas/kas-isar:next
+      - name: Test kas-isar image
+        run: |
+          cd image-tests/isar
+          KAS_IMAGE_VERSION=next ../../kas-container build kas.yml
+      - name: Complete build and deploy kas-isar image
         uses: docker/build-push-action@v2
         with:
           file: Dockerfile.isar
           platforms: linux/amd64,linux/arm64
           build-args: KAS_TAG=next
-          push: true
           tags: ghcr.io/siemens/kas/kas-isar:next
+          push: true