refactor: split out image building job matrix into separate jobs

This will allow using a different runner for each.

.github/actions/build-image/action.yml

@@ -0,0 +1,37 @@
+---
+name: 'build-image'
+
+description: 'builds a dokku docker image'
+
+inputs:
+  architecturet:
+    description: 'platform to build'
+    required: true
+    default: 'linux/amd64'
+
+runs:
+  using: "composite"
+  steps:
+    - uses: actions/checkout@v3
+
+    - name: download packages
+      uses: actions/download-artifact@v3
+      with:
+        name: build
+        path: build
+
+    - name: set up qemu
+      uses: docker/setup-qemu-action@v2
+
+    - name: set up docker buildx
+      id: buildx
+      uses: docker/setup-buildx-action@v2
+
+    - name: build docker image
+      timeout-minutes: 20
+      run: |
+        docker buildx build \
+              --platform ${{ inputs.architecturet }} \
+              --progress plain \
+              --tag "dokku/dokku:latest" \
+              .

.github/workflows/ci.yml

@@ -80,42 +80,38 @@ jobs:
           name: build
           path: build
 
-  build-image:
-    name: build-image.${{ matrix.architecture }}
+  build-image-amd64:
+    name: build-image.linux/amd64
     needs: build
     runs-on: ubuntu-20.04
-    strategy:
-      fail-fast: false
-      matrix:
-        architecture:
-          - linux/amd64
-          - linux/arm
-          - linux/arm64
 
     steps:
-      - uses: actions/checkout@v3
-
-      - name: download packages
-        uses: actions/download-artifact@v3
+      - name: build-image
+        uses: ./.github/actions/build-image
         with:
-          name: build
-          path: build
+          architecture: linux/amd64
 
-      - name: set up qemu
-        uses: docker/setup-qemu-action@v2
+  build-image-arm:
+    name: build-image.linux/arm
+    needs: build
+    runs-on: ubuntu-20.04
 
-      - name: set up docker buildx
-        id: buildx
-        uses: docker/setup-buildx-action@v2
+    steps:
+      - name: build-image
+        uses: ./.github/actions/build-image
+        with:
+          architecture: linux/arm
 
-      - name: build docker image
-        timeout-minutes: 20
-        run: |
-          docker buildx build \
-                --platform ${{ matrix.architecture }} \
-                --progress plain \
-                --tag "dokku/dokku:latest" \
-                .
+  build-image-arm64:
+    name: build-image.linux/arm64
+    needs: build
+    runs-on: ubuntu-20.04
+
+    steps:
+      - name: build-image
+        uses: ./.github/actions/build-image
+        with:
+          architecture: linux/arm64
 
   unit-tests:
     name: unit.${{ matrix.index }}