fix release-slackbot.yml

.github/workflows/release-slackbot.yml

@@ -37,7 +37,7 @@ jobs:
       uses: docker/build-push-action@v2
       with:
         context: .
-        file: {context}/packages/slackbot-proxy/docker/Dockerfile
+        file: packages/slackbot-proxy/docker/Dockerfile
         platforms: linux/amd64
         push: true
         tags: |