composer-normalize: Plugin doesn't work when using http proxy

Steps required to reproduce the problem

1. use proxy for accessing Internet
2. define env vars for using proxy https://gist.github.com/ozh/9751639
3. create project using composer, update some vendors - composer works ok
4. try run composer normalize

Expected Result

composer.json was normalized.

Actual Result

docker-compose exec app composer normalize -vvv
Reading ./composer.json
Loading config file ./composer.json
Checked CA file /etc/ssl/certs/ca-certificates.crt: valid
Executing command (/srv/app): git branch --no-color --no-abbrev -v
Reading /root/.composer/composer.json
Loading config file /root/.composer/composer.json
Reading /srv/app/vendor/composer/installed.json
Reading /root/.composer/vendor/composer/installed.json
Loading plugin PackageVersions\Installer
Loading plugin Symfony\Flex\Flex
Loading plugin Localheinz\Composer\Normalize\NormalizePlugin
Loading plugin Pyrech\ComposerChangelogs\ChangelogsPlugin
Running 1.7.2 (2018-08-16 16:57:12) with PHP 7.2.9 on Linux / 4.15.0-29-generic
Do not run Composer as root/super user! See https://getcomposer.org/root for details
Reading ./composer.json
Loading config file ./composer.json
Executing command (/srv/app): git branch --no-color --no-abbrev -v
Reading /root/.composer/composer.json
Loading config file /root/.composer/composer.json
Reading /srv/app/vendor/composer/installed.json
Reading /root/.composer/vendor/composer/installed.json
Loading plugin PackageVersions\Installer_composer_tmp0
Loading plugin Symfony\Flex\Flex_composer_tmp1
Loading plugin Localheinz\Composer\Normalize\NormalizePlugin_composer_tmp2
Loading plugin Pyrech\ComposerChangelogs\ChangelogsPlugin_composer_tmp3
Reading ./composer.lock
Reading ./composer.lock
Schema URI "https://getcomposer.org/schema.json" does not reference a document that could be read.

see for reference how composer handles HTTP_PROXY env var https://github.com/composer/composer/blob/d5a9d86ee4e934d06073521e630556d4f337629c/src/Composer/Util/StreamContextFactory.php#L43