ci: Push directly from build step

Refactor the workflows so that we can use build-push-action also for
publishing. This is a precondition for switching to buildx, needed for
multi-arch images.

The trick needed for this is using an environment variable to control
whether we should push when building the master branch, initializing it
according to the tag status.

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

.github/workflows/next.yml

@@ -57,17 +57,12 @@ jobs:
       - name: Build kas image
         uses: docker/build-push-action@v2
         with:
-          tags: |
-            ghcr.io/siemens/kas/kas
-            ghcr.io/siemens/kas/kas:next
+          push: true
+          tags: ghcr.io/siemens/kas/kas:next
       - name: Build kas-isar image
         uses: docker/build-push-action@v2
         with:
           file: Dockerfile.isar
-          tags: |
-            ghcr.io/siemens/kas/kas-isar
-            ghcr.io/siemens/kas/kas-isar:next
-      - name: Push images
-        run: |
-          docker push ghcr.io/siemens/kas/kas:next
-          docker push ghcr.io/siemens/kas/kas-isar:next
+          build-args: KAS_TAG=next
+          push: true
+          tags: ghcr.io/siemens/kas/kas-isar:next