tests: split out image building into it's own job to speed up ci

Previously, we would build the dokku package and install it in the same step. While the package building was relatively quick, because of our multi-architecture support, we would end up spending a large amount of time waiting for arm/arm64 images to build.

This change splits those builds out, allowing unit tests to start much quicker and each platform to be tested in parallel.

.github/workflows/ci.yml

@@ -61,6 +61,8 @@ jobs:
 
       - name: build package
         run: ./tests/ci/setup.sh build
+        env:
+         SKIP_IMAGE_BUILD: true
 
       - name: set matrix for build
         id: set-matrix
@@ -75,6 +77,41 @@ jobs:
           name: build
           path: build
 
+  build-image:
+    name: build-image.${{ matrix.architecture }}
+    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
+        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
+        run: |
+          docker buildx build \
+                --platform ${{ matrix.architecture }} \
+                --progress plain \
+                --tag "dokku/dokku:latest"
+
   unit-tests:
     name: unit.${{ matrix.index }}
     needs: build

contrib/release-dokku

@@ -391,7 +391,7 @@ fn-build-docker-image() {
       --push \
       --tag "dokku/dokku:latest" \
       --tag "dokku/dokku:$VERSION" .
-  else
+  elif [[ "$SKIP_IMAGE_BUILD" != "true" ]]; then
     docker buildx build \
       --platform linux/arm,linux/arm64,linux/amd64 \
       --progress plain \