Build docker image once before building cores.

Author: nchapman

.github/workflows/build-release.yml

@@ -9,7 +9,31 @@ permissions:
   contents: write # Required for creating releases and pushing tags
 
 jobs:
+  docker-build:
+    runs-on: sprinters:aws:ubuntu-24.04-arm:c7gd.xlarge:temp=32
+    timeout-minutes: 30
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v5
+        with:
+          submodules: recursive
+          fetch-depth: 0
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Build Docker image (with cache)
+        uses: docker/build-push-action@v6
+        with:
+          context: .
+          load: true
+          tags: lessui-cores-builder:latest
+          cache-from: type=gha
+          cache-to: type=gha,mode=max
+
   build:
+    needs: docker-build
     runs-on: sprinters:aws:ubuntu-24.04-arm:c7gd.xlarge:temp=32
     timeout-minutes: 120 # 2 hours max per architecture
     strategy:
@@ -27,14 +51,13 @@ jobs:
       - name: Set up Docker Buildx
         uses: docker/setup-buildx-action@v3
 
-      - name: Build Docker image (with cache)
+      - name: Build Docker image (from cache)
         uses: docker/build-push-action@v6
         with:
           context: .
           load: true
           tags: lessui-cores-builder:latest
           cache-from: type=gha
-          cache-to: type=gha,mode=max
 
       - name: Clean previous builds
         run: make clean