Update and pin docker/build-push-action

Taken from #160, so the action knows what the 'gha' backend is.

Author: anoadragon453

.github/workflows/docker.yaml

@@ -58,7 +58,7 @@ jobs:
           password: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Build and push all platforms
-        uses: docker/build-push-action@v4
+        uses: docker/build-push-action@263435318d21b8e681c14492fe198d362a7d2c83 # v6.18.0
         with:
           # Always build, but don't push to container registries on PRs.
           push: ${{ github.event_name != 'pull_request' }}