tst+fix: do not save empty tar.gz file + use fastest gzip compression

- Previously, an empty tar.gz file was created on containers 1-3 to avoid errors when persisting to workspace. Container 0 saves the Docker images to tar.gz, but the empty tar.gz file would sometimes overwrite the tar.gz with the Docker images. Consequently, the deploy step would try to load Docker images from an empty tar.gz file and would fail.
- To avoid errors on `persist_to_workspace`, save docker/* instead of docker/nipype-base-latest-py36-py27.tar.gz.

Author: jakubk

.circleci/config.yml

@@ -137,17 +137,13 @@ jobs:
               docker save nipype/nipype:base \
                           nipype/nipype:latest \
                           nipype/nipype:py36 \
-                          nipype/nipype:py27 | gzip -6 > /tmp/docker/nipype-base-latest-py36-py27.tar.gz
+                          nipype/nipype:py27 | gzip -1 > /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
-              # not deploying Docker images.
-              touch /tmp/docker/nipype-base-latest-py36-py27.tar.gz
             fi
       - persist_to_workspace:
           root: /tmp
           paths:
-            - docker/nipype-base-latest-py36-py27.tar.gz
+            - docker/*
 
 
   deploy: