Start dstack-shim service after network-online

[un-def]

Fixes: #1476
See: https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/

$ systemd-analyze critical-chain dstack-shim.service

[...]
dstack-shim.service @3h 2min 49.127s
└─network-online.target @4.379s
  └─NetworkManager-wait-online.service @838ms +3.538s
    └─NetworkManager.service @775ms +53ms
      └─dbus.service @772ms
        └─basic.target @763ms
          └─sockets.target @762ms
[...]