CI: use system docopt-cpp

Change-Id: Ib246d39f975c00bc6489f683f1f21f34cc808201
Depend-on: https://gerrit.cesnet.cz/c/CzechLight/dependencies/+/6009

Author: jktjkt

ci/pre.yaml

@@ -23,3 +23,9 @@
         name: sssd-client
         state: absent
       become: true
+
+    - name: install docopt-cpp
+      package:
+        name: docopt-cpp-devel
+        state: present
+      become: true