Merge branch 'fix/ci-php-docker-example' into 'master'

Fix php-docker example (for new docker versions)

`/.dockerinit` file doesn't exists in new Docker versions. Instead `/.dockerenv` can be used. This MR fixes the PHP example which was working only for older Docker versions.

Fixes gitlab-org/gitlab-ci-multi-runner#1241

See merge request !3907

doc/ci/examples/php.md

@@ -40,7 +40,7 @@ repository with the following content:
 #!/bin/bash
 
 # We need to install dependencies only for Docker
-[[ ! -e /.dockerinit ]] && exit 0
+[[ ! -e /.dockerenv ]] && [[ ! -e /.dockerinit ]] && exit 0
 
 set -xe