memcached: Add a Memcached image

The 10MB of overhead memory were estimated from this output in a
running memcached container:

  # ps aux
  USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
  root         1  0.0  0.0  20300  2124 ?        S    20:57   0:00 /bin/bash
  memcach+   184  0.0  0.0 323124  1112 ?        Ssl  20:58   0:00 /usr/bin/memcached -d -p 11211 -U 11211 -m 128 -c 1024 -u
  root       224  0.0  0.0  25064   440 ?        S    20:58   0:00 supervising syslog-ng
  root       225  0.3  0.0 124708  3620 ?        Ssl  20:58   0:00 /usr/sbin/syslog-ng --persist-file /var/lib/syslog-ng/sysl
  root       263  0.0  0.0  18632   804 ?        Ss   20:58   0:00 /usr/sbin/cron
  root       300  0.0  0.0  15440  1148 ?        R+   20:58   0:00 ps aux

Less Bash and ps, that's 1112 + 3620 + 804 = 5.5MB of resident memory,
so 10MB seemed like a safe buffer.