Fix docker build

Looks like I've broken this in #204

.github/workflows/ci.yml

@@ -79,7 +79,7 @@ jobs:
         if: github.event_name != 'pull_request'
         run: |
           mkdir -p /tmp/digests
-          digest="${{ steps.build.outputs.digest }}"
+          digest="${{ steps.build_push.outputs.digest }}"
           touch "/tmp/digests/${digest#sha256:}"          
       - name: Upload digest
         if: github.event_name != 'pull_request'