ansible/docker: ensure gnupg + crontab are installed

Both are required by docker, but seem to be missing from explicit
dpkg 'Requires' :/

TASK [docker : add docker gpg key to apt keyring] ********************************************************
fatal: [rpi4-deb9build-ansible]: FAILED! => {"changed": false, "msg": "Failed to find required executable gpg in paths: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"}

TASK [docker : cleanup old docker images] ****************************************************************
[WARNING]: The value 0 (type int) in a string field was converted to '0' (type string). If this does not
look like what you expect, quote the entire value to ensure it does not change.

fatal: [rpi4-deb9build-ansible]: FAILED! => {"changed": false, "msg": "Failed to find required executable crontab in paths: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"}

Change-Id: I2d342f85f6bc1b80eaafc7075552019fea147894

ansible/roles/docker/tasks/main.yml

@@ -2,9 +2,13 @@
 
 - name: add https support
   apt:
-    name: apt-transport-https
+    name: "{{ item }}"
     cache_valid_time: 3600
     update_cache: yes
+  with_items:
+    - apt-transport-https
+    - gnupg
+    - cron
 
 - name: add docker gpg key to apt keyring
   apt_key: