Environment Variables are NEVER private. Please don't think that you can hide information in there as all of that information ends up in the process table which is public across the entire machine.
Any environment for a process will be accessible via /proc/<pid>/environ on a linux system. Of course other users cannot read these files, however in the case of something like a Docker image all processes likely share a username and this could be a risk (especially for a public webapp that one day may information leak/allow remote command execution).
At least that's my immediate take on it.