after setting up ssh/passwordless login, install git server through Synology package manager.
SSH into Synology server. Run:
$ pwd
/volume1/homes/<username>/
mkdir git-repos
cd git-repos
git --bare init <repo-name>.git
cd <repo-name>.git
git update-server-info
on client, locally clone the new empty repo:
git clone ssh://<username>@<synology-ip-address>/var/services/homes/<username>/git-repos/<repo-name>.git