Currently SncRedis doesn't allow to use the new environment variables, apparently this bundle does something before the envs are setted. So you can't use redis_dsn: "%env(REDIS_DSN)%" because you receive that literal string instead the current env value.

Info: http://symfony.com/blog/new-in-symfony-3-2-runtime-environment-variables
PR: symfony/symfony#19681

In the PR there is a comment:

magnusnordlander commented on 20 Aug
This is an interesting approach, but it is not as backwards compatible as one would think. There are bundles out there today, which as a part of their extension actually manipulate parameters. SncRedisBundle e.g. splits DSNs in the extension. Passing one of these parameters there will at best cause some kind of syntax error in the bundle, or at worst cause old environment values to be cached.