fix: use gzip compression -6 instead of -1

The lowest amount of compression created a tar.gz file of over 5.0 GB. Persisting this to the CircleCI workspace did not show errors, however, tar.gz file in the next step of the workflow would be empty. This caused `docker load` to fail. More compression leads to a tar.gz file that is about 4.7 GB. This relatively smaller file can be loaded in the next step of the CircleCI workflow.

In a future PR, the Nipype container will be minimized with ReproZip. This will create smaller containers and alleviate the issue that this commit aims to fix.

Author: kaczmarj

.circleci/config.yml

@@ -136,7 +136,7 @@ jobs:
               docker save nipype/nipype:base \
                           nipype/nipype:latest \
                           nipype/nipype:py36 \
-                          nipype/nipype:py27 | gzip -1 > /tmp/docker/nipype-base-latest-py36-py27.tar.gz
+                          nipype/nipype:py27 | gzip -6 > /tmp/docker/nipype-base-latest-py36-py27.tar.gz
               du -h /tmp/docker/nipype-base-latest-py36-py27.tar.gz
             else
               # Workaround for `persist_to_workspace` to succeed when we are