Consider getenv for MW_ELASTIC_HOST

[ross-spencer]

In php.ini $_ENV isn't recommended for production servers.

From an example php.ini:

; This directive determines which super global arrays are registered when PHP
; starts up. G,P,C,E & S are abbreviations for the following respective super
; globals: GET, POST, COOKIE, ENV and SERVER. There is a performance penalty
; paid for the registration of these arrays and because ENV is not as commonly
; used as the others, ENV is not recommended on productions servers. You
; can still get access to the environment variables through getenv() should you
; need to.
; Default Value: "EGPCS"
; Development Value: "GPCS"
; Production Value: "GPCS";
; http://php.net/variables-order
variables_order = "GPCS"

$_ENV is used here:

wikibase-release-pipeline/Docker/build/WikibaseBundle/LocalSettings.d.template/WikibaseCirrusSearch.php

Line 7 in 73f9942

$wgCirrusSearchServers = [ $_ENV['MW_ELASTIC_HOST'] ];

Using a default production php.ini, e.g. from /usr/local/etc/php, e.g. to override file upload images for the local media extension, this will cause errors when trying to switch from CirrusSearch to elastic, i.e. PHP Notice: Undefined index: MW_ELASTIC_HOST in /var/www/html/LocalSettings.d/WikibaseCirrusSearch.php on line 7.

A workaround I guess is to add E to variables_order in php.ini (or perhaps mount a modified WikibaseCirrusSearch.php. If there's a more opinionated view on php.ini from the team, perhaps that can be distributed with this repo?