portage: Add a Portage-volume-exporting image

This builds the Portage snapshot into a stand-alone volume, which can
be mounted using -volumes-from.  There's currently no
Dockerfile-supported way to mount this for builds, so gentoo-portage
still goes the emerge-webrsync route.

We need something to fill the role of /bin/sh in the Portage-snapshot
container, so I'm using the /bin/busybox from the 'gentoo' image.  As
of 2013-12-12, it's:

  sys-apps/busybox-1.21.0
    USE="ipv6 pam static -livecd -make-symlinks -math -mdev -savedconfig
         (-selinux) -sep-usr -syslog -systemd"
build.sh: Allow readlink for path-canonicalization

Debian doesn't have realpath installed by default [1], so fall back to
the more widely available readlink [2].

[1]: http://packages.debian.org/unstable/utils/realpath
[2]: http://www.gnu.org/software/coreutils/manual/html_node/readlink-invocation.html
redis/Dockerfile.template: Bind to all interfaces

It's not much good to have a Redis container that's only listening to
localhost ;).
postgresql/README.md: Mention volume-mounts and setup

Without a host-mounted volume for /var/lib/postgresql I filled up the
10GB dm block allocated by Docker v0.7.2.
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.
Use envsubst instead of sed to parse Dockerfile.template

Convert everything in one pass, instead of taking a separate pass for
each variable.  Thanks to aluedeke for mentioning envsubst on #docker;
I hadn't heard of it before.  It's distributed as part of gettext [1],
and not in POSIX [2], so it may be slightly less portable.  However,
envsubst and gettext are in Gentoo's x86 and amd64 stage1s, so it
can't be *that* uncommon ;).

[1]: http://www.gnu.org/software/gettext/
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/idx/utilities.html
build.sh: Bump DATE to 20131226 to match the new weekly build
redis/Dockerfile.template: Direct logs to syslog

Since I'm tailing /var/log/messages in CMD.  This also makes it easier
if you want to redirect the logs to a central syslog server.
portage/README.md: You can mount volumes from stopped containers

As Sven Dowideit describes in a pending doc update [1].

[1]: https://github.com/dotcloud/docker/pull/3389