tests:pin EL7 for nikola-el8 scenari to workaorund a Docker bug

Author: duck-rh

molecule/nikola-el8/molecule.yml

@@ -1,6 +1,9 @@
 ---
 scenario:
   name: nikola-el8
+# pin centos version to workaround a bug in Docker
+#   https://github.com/docker/for-linux/issues/835
+#   https://github.com/systemd/systemd/issues/11752
 platforms:
   - name: ansible-test-builder
     # LXD
@@ -12,9 +15,9 @@ platforms:
   - name: ansible-test-web
     # LXD
     source:
-      alias: 'centos/8/amd64'
+      alias: 'centos/7/amd64'
     # Docker
-    image: 'centos:8'
+    image: 'centos:7.6.1810'
 provisioner:
   inventory:
     group_vars: