Speed up saving image

Using `pigz` cuts the compression time from about 4 minutes to 2 minutes
in the default 4 core GitHub Actions runner.

The `upload-artifact` action also compresses with `zlib` which is
redundant, so let's disable this to speed up the upload step.

.github/workflows/main.yml

@@ -35,13 +35,15 @@ jobs:
 
       - name: Save docker image
         run: |
-          docker save research-template | gzip > /tmp/research-template.tar.gz
+          docker save research-template | pigz > /tmp/research-template.tar.gz
 
       - name: Upload docker image
         uses: actions/upload-artifact@v4
         with:
             name: research-template-image
             path: /tmp/research-template.tar.gz
+            # Disable compression; the file is already compressed
+            compression-level: 0
 
   publish:
     needs: [build-and-test]