Set OOMScoreAdjust to stop box code from being killed

OOMScoreAdjust can be set between -1000 and +1000. This value is inherited
and systemd has no easy way to control this for children (box code also
runs as non-root, so it cannot easily set it for the children using
/proc/<pid>/oom_score_adj.

When set to -1000 and the process reaches the MemoryMax, it seems the kernel
does not kill any process in the cgroup and it spins up in high memory. In fact,
'systemctl status <service>' stops displaying child process (but ps does), not sure
what is happenning.

Keeping it -999 means that if a child process consumed a lot of memory, the kernel
will kill something in the group. If the main box itself is killed, systemd will
kill it at all because of KillMode=control-group.

Keeping it -999 also saves box service group being killed relative to other docker
processes (apps and addons).

Fixes #605

setup/start/systemd/box.service

@@ -17,9 +17,12 @@ ExecStart=/bin/sh -c 'echo "Logging to /home/yellowtent/platformdata/logs/box.lo
 Environment="HOME=/home/yellowtent" "USER=yellowtent" "DEBUG=box*,connect-lastmile" "BOX_ENV=cloudron" "NODE_ENV=production"
 ; kill apptask processes as well
 KillMode=control-group
+; Do not kill this process on OOM. Children inherit this score. Do not set it to -1000 so that MemoryMax can keep working
+OOMScoreAdjust=-999
 User=yellowtent
 Group=yellowtent
-MemoryLimit=400M
+; OOM killer is invoked in this unit beyond this
+MemoryMax=400M
 TimeoutStopSec=5s
 StartLimitInterval=1
 StartLimitBurst=60