Locale set to en_US.UTF-8

Summary:
If a commit contains an UTF-8 character like → (right arrow),
Arcanist mangles it when using arc diff.

We install the locales package and fix the locale to en_US.UTF-8.

Reference: http://serverfault.com/a/689947/186660 by EirikW

Fixes T611.

Test Plan:
Type an é in a shell launched in the container.

To launch it, arc shell or docker run --rm -it nasqueron/arcanist bash.

Reviewers: Kaliiixx

Subscribers: Nasqueron Docker deployment squad

Projects: Docker images

Maniphest Tasks: T611

Differential Revision: http://devcentral.nasqueron.org/D101