Speedup docker builds

GHA allows 20 concurrent jobs. So make use of it.
https://help.github.com/en/actions/automating-your-workflow-with-github-actions/workflow-syntax-for-github-actions#usage-limits

.github/workflows/blank.yml

@@ -27,7 +27,46 @@ jobs:
         with:
           name: pigallery2-release
           path: release
-  build:
+  build-alpine:
+    runs-on: [ubuntu-latest]
+    needs: [create-release]
+    steps:
+      -
+        name: Checkout
+        uses: actions/checkout@v1
+      -
+        uses: actions/download-artifact@v1
+        with:
+          name: pigallery2-release
+      -
+        name: Set up Docker Buildx
+        id: buildx
+        uses: crazy-max/ghaction-docker-buildx@v1
+        with:
+          version: latest
+      -
+        name: Available platforms
+        run: echo ${{ steps.buildx.outputs.platforms }}
+      -
+        name: Docker login
+        run: |
+          docker login -u "${DOCKER_REGISTRY_USERNAME}" -p "${DOCKER_REGISTRY_PASSWORD}"
+        env:
+          DOCKER_REGISTRY_USERNAME: ${{ secrets.REGISTRY_USERNAME }}
+          DOCKER_REGISTRY_PASSWORD: ${{ secrets.REGISTRY_PASSWORD }}
+      -
+        name: Run Buildx alpine
+        run: |
+          docker buildx build \
+            --platform linux/amd64,linux/arm64,linux/386,linux/arm/v6,linux/arm/v7 \
+            --output "type=image,push=true" \
+            --tag bpatrik/pigallery2:nightly-alpine \
+            --file docker/alpine/Dockerfile.build  .
+      -
+        name: Docker logout
+        run: |
+          docker logout
+  build-stretch:
     runs-on: [ubuntu-latest]
     needs: [create-release]
     steps:
@@ -62,14 +101,6 @@ jobs:
             --output "type=image,push=true" \
             --tag bpatrik/pigallery2:nightly-stretch \
             --file docker/debian-stretch/Dockerfile.build  .
-      -
-        name: Run Buildx alpine
-        run: |
-          docker buildx build \
-            --platform linux/amd64,linux/arm64,linux/386,linux/arm/v6,linux/arm/v7 \
-            --output "type=image,push=true" \
-            --tag bpatrik/pigallery2:nightly-alpine \
-            --file docker/alpine/Dockerfile.build  .
       -
         name: Docker logout
         run: |