try more idiomatic way of building the archive for PR workflow runs

.github/workflows/builder-ubuntu.yml

@@ -22,46 +22,27 @@ jobs:
   ubuntu-builder:
     name: psibase-builder-ubuntu-${{ inputs.ubuntu_version }}
     runs-on: ubuntu-latest
-    services:
-      registry:
-        image: registry:2
-        ports:
-          - 5000:5000
     steps:
       - name: Checkout code
         uses: actions/checkout@v4
         with:
           submodules: false
           fetch-depth: 0
 
-      - name: Free up runner space
-        run: bash ${GITHUB_WORKSPACE}/.github/scripts/free-disk-space.sh
-
       - name: Preparation
         id: prep
         run: |
           OWNER="${{ github.repository_owner }}"
           IMAGE="psibase-builder-ubuntu-${{ inputs.ubuntu_version }}"
-          if [[ "${{ inputs.is_pr }}" == "true" ]]; then
-            REGISTRY="localhost:5000"
-            TAG="latest"
-          else
-            REGISTRY="ghcr.io"
-            TAG="${{ github.sha }}"
-          fi
+          REGISTRY="ghcr.io"
+          TAG="${{ github.sha }}"
+
           TAGS="${REGISTRY}/${OWNER}/${IMAGE}:${TAG}"
           echo "tags=${TAGS,,}" >> $GITHUB_OUTPUT
 
       - name: Building ${{ steps.prep.outputs.tags }}
         run: true
 
-      - name: (PR only) - Config docker buildx network
-        if: ${{ inputs.is_pr }}
-        uses: docker/setup-buildx-action@v2
-        with:
-          buildkitd-flags: --debug
-          driver-opts: network=host
-
       - name: Login in to registry
         if: ${{ !inputs.is_pr }}
         uses: docker/login-action@v2
@@ -71,6 +52,7 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Build & Publish Image
+        if: ${{ !inputs.is_pr }}
         uses: docker/build-push-action@v4
         with:
           context: .
@@ -79,14 +61,18 @@ jobs:
           tags: ${{ steps.prep.outputs.tags }}
           platforms: linux/amd64
           outputs: type=image,annotation-index.org.opencontainers.image.description=Psibase build environment based on Ubuntu ${{ inputs.ubuntu_version }}
-
-      - name: (PR only) - Save docker image to archive
+    
+      - name: (PR Only) - Build image archive
         if: ${{ inputs.is_pr }}
-        run: |
-          docker pull ${{ steps.prep.outputs.tags }}
-          docker save ${{ steps.prep.outputs.tags }} -o builder-${{ inputs.ubuntu_version }}-image.tar
+        uses: docker/build-push-action@v4
+        with:
+          context: .
+          file: docker/ubuntu-${{ inputs.ubuntu_version }}-builder.Dockerfile
+          tags: ${{ steps.prep.outputs.tags }}
+          platforms: linux/amd64
+          outputs: type=docker,dest=builder-${{ inputs.ubuntu_version }}-image.tar
 
-      - name: (PR only) - Upload docker image archive
+      - name: (PR only) - Upload image archive as artifact
         if: ${{ inputs.is_pr }}
         uses: actions/upload-artifact@v4
         with: