Installation not working behind a proxy

[Finulin]

The installation does not work behind a corporate proxy.
Here is the error message:

  - Installing phar-io/filesystem (2.0.1): Extracting archive
  - Installing phar-io/executor (1.0.1): Extracting archive
  - Installing phar-io/gnupg (1.0.2): Extracting archive
  - Installing phar-io/composer-distributor (1.0.2): Extracting archive
  - Installing captainhook/captainhook-phar (5.23.3): Extracting archive
  - Downloading artifact from https://github.com/captainhookphp/captainhook/releases/download/5.23.3/captainhook.phar

In Download.php line 26:
                                                                                                   
  fopen(): php_network_getaddresses: getaddrinfo for github.com failed: Name or service not known 

Composer and environment variables are configured correctly, my Ubuntu works fine:

# env
NO_PROXY=localhost,127.0.0.1,::1
FTP_PROXY=webproxy.is.elb.de:9090
SHLVL=1
HTTPS_PROXY=webproxy.is.elb.de:9090
HTTP_PROXY=webproxy.is.elb.de:9090

# composer diag
Checking platform settings: OK
Checking git settings: OK git version 2.25.1
Checking http connectivity to packagist: OK
Checking https connectivity to packagist: OK
Checking HTTP proxy with http: OK http://webproxy.is.elb.de:9090
Checking HTTP proxy with https: OK http://webproxy.is.elb.de:9090
Checking github.com oauth access: OK does not expire
Checking disk free space: OK

Does anyone have any idea what I can do?