docker ubuntu /bin/sh: 1: locale-gen: not found

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.