Use local context for docker build to support Merge Queue

* Updates docker.yml to set the build context to the
  local directory.
* This prevents "ref not found" errors when the
  workflow runs on temporary branches generated by
  the GitHub Merge Queue.
* Ensures the Docker builder utilizes the files
  already present in the runner's workspace from
  the checkout step.

.github/workflows/docker.yml

@@ -30,6 +30,7 @@ jobs:
       id-token: write
       attestations: write
     with:
+      context: . # Use local checkout to support GitHub Merge Queue
       output: image
       platforms: linux/amd64,linux/arm64
       push: ${{ github.event_name != 'pull_request' }}