set "check_mode: no" for read-only "shell" steps that registers result

"shell" step doesn't support check mode, which currently leads to failures,
when Ansible is being run in check mode (because Ansible doesn't run command,
assuming that command might have effect, and no "rc" or "output" is registered).

Setting "check_mode: no" allows to run those "shell" commands in check mode
(which is safe, because those shell commands doesn't have side effects).

roles/docker/tasks/set_facts_dns.yml

@@ -32,11 +32,13 @@
   shell: grep "^nameserver" /etc/resolv.conf | sed 's/^nameserver\s*//'
   changed_when: False
   register: system_nameservers
+  check_mode: no
 
 - name: check system search domains
   shell: grep "^search" /etc/resolv.conf | sed 's/^search\s*//'
   changed_when: False
   register: system_search_domains
+  check_mode: no
 
 - name: add system nameservers to docker options
   set_fact: