Setting C.UTF-8 as default environment variable

Summary:
According to https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=636086,
using en_US.UTF-8 is deprecated.

Setting only LANG variable, given that it overrides other environment
variables (http://pubs.opengroup.org/onlinepubs/7908799/xbd/envvar.html).

Resolves T1024.

Test Plan: Rebuild the image's tags on Dwellers, echo $LANG

Reviewers: dereckson

Subscribers: Nasqueron Docker deployment squad

Tags: Docker images

Maniphest Tasks: T1025, T1024

Differential Revision: https://devcentral.nasqueron.org/D685