systemd: set OOMScoreAdjust for dockerd

dockerd currently sets the oom-score-adjust itself. This functionality
was added when we did not yet run dockerd as a systemd service.

Now that we do, it's better to instead have systemd handle this.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

contrib/init/systemd/docker.service

@@ -42,6 +42,7 @@ Delegate=yes
 
 # kill only the docker process, not all processes in the cgroup
 KillMode=process
+OOMScoreAdjust=-500
 
 [Install]
 WantedBy=multi-user.target