Ensure OOM killer is disabled in the postgis image

[strk]

The OOM killer brings down a while PostgreSQL cluster rather than cleanly report out-of-memory condition. Need be disabled, after testing how it hurts.

For disabling: http://serverfault.com/questions/101916/turn-off-the-linux-oom-killer-by-default

For testing without harming any useful process, see #61