add memcached support

[WolfgangFahl]

use WolfgangFahl/pymediawikidocker#102 to do so

add service to docker-compose.yaml:

  memcached:
    image: memcached:alpine
    restart: always

add config to Localsettings.php

$wgMainCacheType = CACHE_MEMCACHED;
$wgParserCacheType = CACHE_MEMCACHED; 
$wgMessageCacheType = CACHE_MEMCACHED; 
$wgMemCachedServers = [ 'memcached:11211' ];
$wgSessionsInObjectCache = true; 
$wgSessionCacheType = CACHE_MEMCACHED;