Update image setup docs - disabling services

doc/image_setup_checklist.md

@@ -38,6 +38,7 @@
 * Symlink /proc/mounts to /etc/mtab
 * Turn on rc-local.service `systemctl enable rc-local.service`
     - If it doesn't work, read these instructions: https://www.linuxbabe.com/linux-server/how-to-enable-etcrc-local-with-systemd
+* Disable networking.service, bluetooth.service, and dchpcd.service
 * apt-get autoclean
 * apt-get autoremove
 * Exit from proot

doc/setup_with_proot.md

@@ -185,6 +185,13 @@ This is what triggers CIRCLean to run.
 ```
     systemctl enable rc-local.service
 ```
+* Turn off several networking related services. This speeds up boot and reduces the
+attack surface:
+```
+    systemctl disable networking.service
+    systemctl disable bluetooth.service
+    systemctl disable dhcpcd.service
+```
 * Clean up:
 ```
     apt-get clean