feat: minor improvements in user setup and configuration (#34)

1. Avoid conflicts with extra groups by creating them before installing system dependencies
2. Enable ssh signature in git

Reviewed-on: #34
Co-authored-by: Joao P Dubas <joao.dubas+gitea@gmail.com>
Co-committed-by: Joao P Dubas <joao.dubas+gitea@gmail.com>

config/git/.exists

@@ -0,0 +1,9 @@
+[commit]
+	gpgsign = true
+[gpg]
+	format = ssh
+[user]
+	email = {{email}}
+	name = {{name}}
+	signingKey = {{path-to-pub-ssh-key}}
+