Right now Docker relies on PHP_IMAGE_DEV to install Composer dependencies:
https://github.com/ezsystems/ezplatform/blob/2.5/doc/docker/install-dependencies.yml#L6

This is misleading, as it's not enough to specify PHP_IMAGE to use the same PHP version across our Docker stack - leading to failures like this one: https://travis-ci.org/github/ezsystems/ezplatform-http-cache/builds/674863003 because only PHP_IMAGE variable is specified: https://github.com/ezsystems/ezplatform-http-cache/blob/1.0/.travis.yml#L26

We should change the install_dependencies container so that it relies on PHP_IMAGE