Use unicode locale by default

[nigredo-tori]

At the moment the system (and, consequently, Postgres) uses a POSIX locale. This is problematic for users that use languages other than English. For example, I've created a database with default encoding, and that screwed up ILIKE in my queries way down the line.

I think that Unicode should be the default encoding in this image. Adding something like this to the Dockerfile should be enough:

RUN locale-gen en_US.UTF-8
ENV LANG en_US.UTF-8 

UPD: see this SO answer for some explanation.
UPD2: The official PG image also uses UTF locale by default.