nginx-proxy: Add an Nginx proxy image

nginx-proxy: Add an Nginx proxy image

I've got a bunch of HTTP server images, but I need a single proxy
container that supports SNI to expose them to the world.  This
container can link to a colleciton of HTTP-serving containers and
proxy them under arbitrary hostnames using user-supplied, per-host
certificates and keys.

env and sed are both in POSIX 2013 [1,2], and the:

  eval A=\$$B

variable indirection is because POSIX [3] doesn't support Bash's
indirect expansion "A=${!B}".  The 'tcp' -> 'http' replacement maps
Docker's tcp:://${IP}:${PORT} addresses to http://${IP}:${PORT} for
use in the Nginx config.

envsubst is in gettext [4] and not in POSIX [5], so it may be slightly
less portable.  However, envsubst and gettext are in Gentoo's x86 and
amd64 stage1s, our Gentoo-based images will have it.

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/env.html
[2]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/sed.html
[3]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02
[4]: http://www.gnu.org/software/gettext/
[5]: http://pubs.opengroup.org/onlinepubs/9699919799/idx/utilities.html