ci: fix `hardhat-node` docker publish

There was a bug in the config, the `file`
was not relative to the `context`

.github/workflows/publish-canary.yml

@@ -167,7 +167,7 @@ jobs:
         uses: docker/build-push-action@v2
         with:
           context: ./ops/docker/hardhat
-          file: ./Dockerfile
+          file: ./ops/docker/hardhat/Dockerfile
           push: true
           tags: ethereumoptimism/hardhat-node:${{ needs.canary-publish.outputs.canary-docker-tag }}