refactor(ansible): centralize inventory and variables in 'vars' directory

Signed-off-by: Tuan-Dat Tran <tuan-dat.tran@tudattr.dev>

vars/group_vars/docker/vars.yml

@@ -0,0 +1,8 @@
+caddy:
+  admin_email: me+acme@tudattr.dev
+
+domain: "{{ internal_domain }}"
+
+netcup_api_key: "{{ vault.netcup.api_key }}"
+netcup_api_password: "{{ vault.netcup.api_password }}"
+netcup_customer_id: "{{ vault.netcup.customer_id }}"