Document Atlas backend phase one and WireGuard deployment

ansible/roles/profile_backend_phase1/templates/atlas-syncthing.container.j2

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+# Managed by Ansible. Do not edit manually.
+[Unit]
+Description=Atlas rootless Syncthing backend
+
+[Container]
+ContainerName=atlas-syncthing
+Image={{ backend_phase1_syncthing_image }}
+HostName=atlas-syncthing
+Network=host
+UserNS=keep-id
+User={{ backend_phase1_uid }}
+Group={{ backend_phase1_gid }}
+Environment=HOME=/var/syncthing
+Environment=STHOMEDIR=/var/syncthing/config
+Environment=STGUIADDRESS={{ backend_phase1_wireguard_address }}:{{ backend_phase1_syncthing_gui_port }}
+Environment=TZ={{ backend_phase1_timezone }}
+Volume={{ backend_phase1_syncthing_root }}:/var/syncthing:Z
+NoNewPrivileges=true
+DropCapability=all
+
+[Service]
+Restart=always
+RestartSec=10
+TimeoutStartSec=900
+
+[Install]
+WantedBy=default.target