local_exporter: Start after network online

The *local_exporter.service* cannot start on first boot without the
network, as it needs to pull the container image from.

local_exporter.container

@@ -1,6 +1,7 @@
 [Unit]
 Description=Bridge for local Prometheus metrics
-After=network.target
+After=network-online.target
+Wants=network-online.target
 
 [Container]
 Image=git.pyrocufflink.net/containerimages/local_exporter:latest