Set TasksMax in addition to LimitNPROC in systemd service files

systemd sets an additional limit on processes and threads that defaults to 512 when run under Linux >= 4.3.
See more information here: http://unix.stackexchange.com/a/255603/59955

Signed-off-by: Candid Dauth <cdauth@cdauth.eu>

contrib/init/systemd/docker.service

@@ -11,6 +11,7 @@ MountFlags=slave
 LimitNOFILE=1048576
 LimitNPROC=1048576
 LimitCORE=infinity
+TasksMax=1048576
 TimeoutStartSec=0
 
 [Install]

docs/articles/systemd.md

@@ -88,6 +88,7 @@ In this example, we'll assume that your `docker.service` file looks something li
     ExecStart=/usr/bin/docker daemon -H fd://
     LimitNOFILE=1048576
     LimitNPROC=1048576
+    TasksMax=1048576
 
     [Install]
     Also=docker.socket