title: composer install gives errors on Gitlab CI
body: <p>I try to run <em>composer install</em> command on Gitlab runner. I would like to install dependencies first of all on runner. But I am getting following error. When I try on my local machine, I install all dependencies without error.</p>
  
  <pre><code>Updating dependencies
  Your requirements could not be resolved to an installable set of packages.
  
    Problem 1
      - don't install symfony/security-guard 4.1.x-dev|don't install symfony/symfony v3.4.17
      - don't install symfony/security-guard 4.2.x-dev|don't install symfony/symfony v3.4.17
      - don't install symfony/security-guard v4.1.0|don't install symfony/symfony v3.4.17
      - don't install symfony/security-guard v4.1.0-BETA1|don't install symfony/symfony v3.4.17
      - don't install symfony/security-guard v4.1.0-BETA2|don't install symfony/symfony v3.4.17
      - don't install symfony/security-guard v4.1.0-BETA3|don't install symfony/symfony v3.4.17
      - don't install symfony/security-guard v4.1.1|don't install symfony/symfony v3.4.17
      - don't install symfony/security-guard v4.1.2|don't install symfony/symfony v3.4.17
      - don't install symfony/security-guard v4.1.3|don't install symfony/symfony v3.4.17
      - don't install symfony/security-guard v4.1.4|don't install symfony/symfony v3.4.17
      - don't install symfony/security-guard v4.1.5|don't install symfony/symfony v3.4.17
      - don't install symfony/security-guard v4.1.6|don't install symfony/symfony v3.4.17
      - Installation request for symfony/symfony 3.4.17 -&gt; satisfiable by symfony/symfony[v3.4.17].
      - Installation request for symfony/security-guard ^4.1 -&gt; satisfiable by symfony/security-guard[4.1.x-dev, 4.2.x-dev, v4.1.0, v4.1.0-BETA1, v4.1.0-BETA2, v4.1.0-BETA3, v4.1.1, v4.1.2, v4.1.3, v4.1.4, v4.1.5, v4.1.6].
  </code></pre>
  
  <p>My Gitlab CI file;</p>
  
  <pre><code>image: php:7.1
  
  variables:
    ENV: master
    LABEL: latest
    DOCKER_DRIVER: overlay
  stages:
  - master
  
  services:
  - docker:dind
  
  before_script:
  - apt-get update
  - apt-get install -y zip unzip wget apt-transport-https curl software-properties-common gnupg2 libmcrypt-dev
  - apt-get install -y make ca-certificates openssl python
  - apt-get install -y libxml2-dev --no-install-recommends
  - docker-php-ext-install bcmath xml mbstring
  - docker-php-ext-install mysqli pdo pdo_mysql
  - export COMPOSER_MEMORY_LIMIT=-1
  - export COMPOSER_ALLOW_SUPERUSER=1
  - php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
  - php composer-setup.php
  - php -r "unlink('composer-setup.php');"
  - mv composer.phar /usr/local/bin/composer
  
  build:
    stage: master
    script:
    - composer install
    - cd res/deployment/
    - sh build.sh
    only:
    - master
  </code></pre>