Community maintained Docker config for the spindle server

Add `authorized_keys_command.conf` with correct git directory

I couldn't push or pull from any repositories
until I set `-git-dir` following the instructions in
[knot-hosting](https://tangled.sh/@tangled.sh/core/blob/master/docs/knot-hosting.md#repositories),
so this adds the `/etc/ssh/sshd_config.d/authorized_keys_command.conf` file to `rootfs`.

Changed files
+3
rootfs
etc
ssh
+3
rootfs/etc/ssh/sshd_config.d/authorized_keys_command.conf
···
+
Match User git
+
AuthorizedKeysCommand /usr/bin/knot keys -o authorized-keys -git-dir /home/git/repositories
+
AuthorizedKeysCommandUser nobody