feat(infra): Publish Superset docker image on pushes to main (#173)

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## Summary by CodeRabbit

* **Chores**
* Enhanced release management by restricting Docker image publishing and
artifact attestation to the main branch only, ensuring controlled and
stable deployment processes.

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settings.</sub>

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Author: martyngigg

.github/workflows/docker-superset.yml

@@ -20,7 +20,7 @@ env:
   REGISTRY: ghcr.io
   ORG_NAME: isisneutronmuon
   IMAGE_NAME: analytics-data-platform-superset
-  PUBLISH: false
+  PUBLISH: ${{ github.ref == 'refs/heads/main' }}
 
 jobs:
   build-and-push-image: