I actually happened to have suffered from the same problem, but none of the provided answers are 100% working with debian:latest, even if they provide good hints.
The biggest difference is that you should make sure both locales and locales-all are installed, the latter already containing en_US.UTF-8, so you don't have to generate it with local-gen or dpkg-reconfigure.
Here's what I've done in my Dockerfile to make it work:
FROM debian:latest
RUN apt-get update
RUN apt-get install -y locales locales-all
ENV LC_ALL en_US.UTF-8
ENV LANG en_US.UTF-8
ENV LANGUAGE en_US.UTF-8