Feature: Improved variant names and builds

[vovimayhem]

These are the changes I propose for the project's layout:

• Removed the variant based on centos6, as in the end it's redundant with the 'slim' variant.
• Directory layout follows the convention-ish pattern of:
  • MAJOR_VERSION/Dockerfile for the 'standard' variant
  • MAJOR_VERSION/slim/Dockerfile for the 'slim' variant
  • MAJOR_VERSION/onbuild/Dockerfile for the 'onbuild' variant
• Renamed the 'esl' variant folders to 'slim', which the common term used for "a slim image without build tools".
• Refactored the 'slim' variant Dockerfiles for 17 & 18, by basing from the plain official debian:jessie image, then running a commonly-practiced "install build deps, build, then purge the build deps".
• Fixed the 'standard' variants for 17 & 18, as the ODBC libraries were missing, thus Erlang/OTP wasn't being compiled with the ODBC module.

[c0b]

is that moving OTP_DOWNLOAD_SHA1 to top level ENV a better practice? that will be visible to spawned containers, though many images are doing that way, I don't see a necessary case

wow, looks good for everything else

[vovimayhem]

I'm not a fan of it. Actually I'm not a fan of persisting the OTP_VERSION variable also. But is common practice... I can change that (evil grin) without problems. What do you think?

[vovimayhem]

Actually, let's do it :) Less layers can't hurt!

[c0b]

from erlang code the erlang:system_info(otp_release). can only return "18", there's no way to tell the minor version, or I am not aware of any; so keep OTP_VERSION is still useful in some cases, but exposing OTP_DOWNLOAD_SHA1 is no use to applications, I think we can keep it in build time only

the image size 283.6 MB is cool, bro 👍

$ docker images
REPOSITORY       TAG              IMAGE ID            CREATED             VIRTUAL SIZE
otp              18.1-slim        f2811e055447        2 minutes ago       283.6 MB

docker run -it --rm otp:18.1-slim
Erlang/OTP 18 [erts-7.1] [source] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false]

Eshell V7.1  (abort with ^G)
1> erlang:system_info(otp_release).
"18"

docker run -it --rm otp:18.1-slim /bin/bash
root@8952b5d57639:/# env
OTP_VERSION=18.1.3
HOSTNAME=8952b5d57639
TERM=xterm
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
OTP_DOWNLOAD_SHA1=981e6c03c0a310483e6c14edb7dd2acaa842e2f8
PWD=/
LANG=C.UTF-8
SHLVL=1
HOME=/root
_=/usr/bin/env

[c0b]

and LANG=C.UTF-8 is because of some warning when I ran riak, I am now thinking we can also remove that from the base, and keep ENV as minimum as possible.