Dockerfile: Rework generation of kas images

While kas and kas-isar surely have a common ground like the kas tool
itself, kas-isar does not need all the toolchain packages that are
essential Yocto dependencies. Splitting up the images earlier allows to
shrink kas-isar by almost 400 MB.

Use this chance to model both images as different build stages of the
same Dockerfile. That is simpler than creating a temporary "kas-base"
image via a separate Dockerfile.

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

.github/workflows/master.yml

@@ -41,7 +41,7 @@ jobs:
       - name: Build kas-isar image
         uses: docker/build-push-action@v2
         with:
-          file: Dockerfile.isar
+          target: kas-isar
           platforms: linux/amd64,linux/arm64
           push: ${{ env.PUSH_MASTER }}
           tags: ghcr.io/siemens/kas/kas-isar

.github/workflows/next.yml

@@ -96,9 +96,8 @@ jobs:
       - name: Build kas-isar image
         uses: docker/build-push-action@v2
         with:
-          file: Dockerfile.isar
+          target: kas-isar
           load: true
-          build-args: KAS_TAG=next
           tags: ghcr.io/siemens/kas/kas-isar:next
       - name: Test kas-isar image
         run: |
@@ -107,8 +106,7 @@ jobs:
       - name: Complete build and deploy kas-isar image
         uses: docker/build-push-action@v2
         with:
-          file: Dockerfile.isar
+          target: kas-isar
           platforms: linux/amd64,linux/arm64
-          build-args: KAS_TAG=next
           tags: ghcr.io/siemens/kas/kas-isar:next
           push: true

.github/workflows/release.yml

@@ -36,7 +36,7 @@ jobs:
       - name: Build kas-isar image
         uses: docker/build-push-action@v2
         with:
-          file: Dockerfile.isar
+          target: kas-isar
           platforms: linux/amd64,linux/arm64
           push: true
           tags: |