Use git for fetching the geerlingguy.docker role

With this change, all dependency roles are downloaded
using the same mechanism (git), which makes life simpler for tools like
https://gitlab.com/etke.cc/int/agru

requirements.yml

@@ -1,6 +1,7 @@
 ---
 
-- src: geerlingguy.docker
+- src: git+https://github.com/geerlingguy/ansible-role-docker
+  name: geerlingguy.docker
   version: 6.1.0
 
 - src: git+https://github.com/devture/com.devture.ansible.role.docker_sdk_for_python.git