Update Docker image tag to preview in check formatting action

Author: ahosgood

.github/actions/check-formatting/action.yml

@@ -4,5 +4,5 @@ runs:
   using: "composite"
   steps:
     - name: Check formatting
-      run: docker run --rm -v "$(pwd)":/app/ ghcr.io/nationalarchives/tna-python-dev:latest checkformat
+      run: docker run --rm -v "$(pwd)":/app/ ghcr.io/nationalarchives/tna-python-dev:preview checkformat
       shell: bash