Pinning docker-py version to 2.6.1

Change-Id: I90b59d0e834ea7978852e524ced35f2848046ffc
(cherry picked from commit 3215dcb62f8e45df2244fd1b1a2696706c187988)
diff --git a/roles/docker-install/tasks/main.yml b/roles/docker-install/tasks/main.yml
index b758d3e..f1365f4 100644
--- a/roles/docker-install/tasks/main.yml
+++ b/roles/docker-install/tasks/main.yml
@@ -61,7 +61,7 @@
   pip:
     name: "{{ item }}"
   with_items:
-    - docker>=2.4.2
+    - docker==2.6.1
     - docker-compose==1.15.0
     - gitpython
     - graphviz