using envsubst in Dockerfile

Dockerfile

FROM ubuntu:trusty
RUN \
  apt-get update \
  && apt-get -y install gettext-base \
  && apt-get clean \
  && rm -rf /var/lib/apt/lists/*

ENV VALUE=foo
ENV VALUE1=boo
COPY config.txt source_config.txt
RUN envsubst < "source_config.txt" > "config.txt"

You can run it, but must have it in the same line - so they will be in the same context.

RUN export VALUE=foo && \
    export VALUE1=$(cmd to get value1) && \
    envsubst < "source_config.txt" > "config.txt"

Update available packages in Debian

RUN apt-get update

CMD export VALUE=foo &&
export VALUE1=$(cmd to get value1) &&
envsubst < "source_config.txt" > "config.txt"