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

I put the locale setting codes below into my dockerfile,

FROM node:4-onbuild# Set the localeRUN locale-gen en_US.UTF-8ENV LANG en_US.UTF-8ENV LANGUAGE en_US:enENV LC_ALL en_US.UTF-8

but it gives me the error

/bin/sh: 1: locale-gen: not foundThe command '/bin/sh -c locale-gen en_US.UTF-8' returned a non-zero code: 127

any idea?