stunnel: Add a stunnel-wrapper image

Make it easy to wrap arbitrary ports with stunnel.

The explicit pid configuration avoids errors like:

  ... stunnel: ... Cannot create pid file /var/lib/run/stunnel/stunnel.pid
  ... stunnel: ... create: No such file or directory (2)

which is probably just a wart in Gentoo's stunnel packaging.
nginx: Add an Nginx image
kibana: Add a Kibana image

Dump a Kibana tarball into Nginx's htdocs, and use an
ELASTICSEARCH_URL environment variable to adjust config.js at spin-up.

List Kibana after Nginx in build.sh's REPOS to ensure the Nginx image
is built first.
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.