How to set the locale inside a Debian/Ubuntu Docker container?

Those who use Debian also have to install locales package.

RUN apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y locales

RUN sed -i -e 's/# en_US.UTF-8 UTF-8/en_US.UTF-8 UTF-8/' /etc/locale.gen && \
    dpkg-reconfigure --frontend=noninteractive locales && \
    update-locale LANG=en_US.UTF-8

ENV LANG en_US.UTF-8 

This answer helped me a lot.

Put in your Dockerfile something adapted from

# Set the locale
RUN sed -i '/en_US.UTF-8/s/^# //g' /etc/locale.gen && \
    locale-gen
ENV LANG en_US.UTF-8  
ENV LANGUAGE en_US:en  
ENV LC_ALL en_US.UTF-8     

If you run Debian or Ubuntu, you also need to install locales to have locale-gen with

apt-get -y install locales

this is extracted from the very good post on that subject, from

http://jaredmarkell.com/docker-and-locales/