Build image using metadata-action

This still builds the `:main` image via `ref=branch`, since that's in
use in places, but we now also build an `:edge` image that we can
migrate to, as is more conventional.

.github/workflows/ci.yml

@@ -10,38 +10,36 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - id: prep
-        run: |
-          image=restyled/sdk:$(echo "${{ github.sha }}" | head -c7)
-
-          if ${{ github.ref == 'refs/heads/main' }}; then
-            echo "::set-output name=tags::${image},restyled/sdk:main"
-          else
-            echo "::set-output name=tags::${image}"
-          fi
+      - id: meta
+        uses: docker/metadata-action@v4
+        with:
+          images: restyled/sdk
+          tags: |
+            type=edge
+            type=ref,event=branch
+            type=sha,prefix=
 
-      - id: buildx
-        uses: docker/setup-buildx-action@v1
+      - uses: docker/setup-buildx-action@v2
 
-      - uses: actions/cache@v2
+      - uses: actions/cache@v3
         with:
           path: /tmp/.buildx-cache
           key: ${{ runner.os }}-image-${{ github.sha }}
           restore-keys: |
             ${{ runner.os }}-image-
 
-      - uses: docker/login-action@v1
+      - uses: docker/login-action@v2
         with:
           username: ${{ secrets.DOCKERHUB_USERNAME }}
           password: ${{ secrets.DOCKERHUB_PASSWORD }}
 
-      - uses: docker/build-push-action@v2
+      - uses: docker/build-push-action@v4
         with:
-          builder: ${{ steps.buildx.outputs.name }}
           cache-from: type=local,src=/tmp/.buildx-cache
           cache-to: type=local,mode=max,dest=/tmp/.buildx-cache-new
-          push: true
-          tags: ${{ steps.prep.outputs.tags }}
+          push: ${{ github.event_name != 'pull_request' }}
+          tags: ${{ steps.meta.outputs.tags }}
+          labels: ${{ steps.meta.outputs.labels }}
 
       # Avoids ever-growing cache hitting limits
       - run: |