fix: ensure sing-box service wait for network online

[everyx]

Using a DNS server of type dhcp on Linux may lead to the following error during startup, we should ensure sing-box service wait for network online by adding network-online.target dependency:

ERROR[0000] network: missing default interface
ERROR[0000] dns/dhcp[local]: fetch DNS servers: dhcp: prepare interface: missing default interface

[nekohasekai]

Is this fix really necessary? The error you mentioned starts in the background, doesn't block the startup process, and sing-box should automatically reacquire DHCP servers after a change in network status. If it's not working correctly, you should open a new issue.

[everyx]

Is this fix really necessary? The error you mentioned starts in the background, doesn't block the startup process, and sing-box should automatically reacquire DHCP servers after a change in network status. If it's not working correctly, you should open a new issue.

@nekohasekai Well, it doesn't seem appropriate to add unnecessary boot blocking to the system just to eliminate some harmless logs, so I'll close this PR