我的docker构建失败,出现以下错误：

Untar fork/exec /usr/bin/docker: cannot allocate memory

所以我得到它 – 没有RAM :)但我可以清楚地看到空闲内存作为交换：

             total       used       free     shared    buffers     cached
Mem:          7517       7334        183          0         59        515
-/+ buffers/cache:       6759        758
Swap:         4095       2635       1460

此外,似乎docker占用了一个巨大的内存块：

$top -a
top - 03:48:07 up 18 days,17 min,2 users,load average: 1.88,0.93,0.42
Tasks: 127 total,2 running,125 sleeping,0 stopped,0 zombie
cpu(s): 49.9%us,14.0%sy,0.0%ni,13.7%id,22.2%wa,0.0%hi,0.2%si,0.0%st
Mem:   7697592k total,6351288k used,1346304k free,138804k buffers
Swap:  4194300k total,310324k used,3883976k free,1449268k cached

  PID USER      PR  NI  VIRT  RES  SHR S %cpu %MEM    TIME+  COMMAND
 1340 root      20   0 3994m 2.1g 7548 S  6.6 28.8  64:20.43 docker
 5318 root      20   0 1499m 724m 4368 S 65.5  9.6   0:08.80 ruby
  978 root      20   0 1010m 616m   32 S  0.0  8.2   0:27.24 ruby

所以问题是：
为什么不“docker build”使用交换区域作为额外的RAM并决定杀死构建？