Since Symfony 3.4, it's now possible to parse environment variables using %env()% tool: https://goo.gl/8Zsed6

But in some environment, I don't have the right environment variables set, so the application is not bootable (on Insight analysis for instance). To fix it, I need to define the environment variable on a parameter key:

parameters:
    db_hostname: '%env(DB_HOSTNAME)%'
    env(DB_HOSTNAME): db

doctrine:
    dbal:
        host: '%db_hostname%'

It should be interesting to parse a default value directly on the %env()% tool:

doctrine:
    dbal:
        host: '%env(default:db:DB_HOSTNAME)%'