I put the locale setting codes below into my dockerfile,
FROM node:4-onbuild
# Set the locale
RUN locale-gen en_US.UTF-8
ENV LANG en_US.UTF-8
ENV LANGUAGE en_U
Thanks for your comment, edwinksl. I updated my dockerfile below which solved the locale-gen error:
FROM node:4-onbuild
# Set the locale
RUN apt-get clean && apt-get update && apt-get install -y locales
RUN locale-gen en_US.UTF-8
apt-get install -y locales
localedef -i en_US -c -f UTF-8 -A /usr/share/locale/locale.alias en_US.UTF-8
localedef is also good to use.