Use faster compression option

One-off comparison in GitHub Actions with the default 4 core runner:

* default `pigz`: 1.7G file size, 1m 49s
* `--fast`: 1.9G file size, 1m 16s

The upload and download time for the file compressed with default
settings is about 10 seconds (turning compression off for the upload).

.github/workflows/main.yml

@@ -35,7 +35,7 @@ jobs:
 
       - name: Save docker image
         run: |
-          docker save research-template | pigz > /tmp/research-template.tar.gz
+          docker save research-template | pigz --fast > /tmp/research-template.tar.gz
 
       - name: Upload docker image
         uses: actions/upload-artifact@v4