Set ENV in Dockerfile with string interpolation

gistfile1.txt

# DOCKER-VERSION 0.4.8

FROM ubuntu:12.04

# install essentials
RUN apt-get install -y -q git

# Install rbenv
RUN git clone https://github.com/sstephenson/rbenv.git /usr/local/rbenv
RUN echo '# rbenv setup' > /etc/profile.d/rbenv.sh
RUN echo 'export RBENV_ROOT=/usr/local/rbenv' >> /etc/profile.d/rbenv.sh
RUN echo 'export PATH="$RBENV_ROOT/bin:$PATH"' >> /etc/profile.d/rbenv.sh
RUN echo 'eval "$(rbenv init -)"' >> /etc/profile.d/rbenv.sh
RUN chmod +x /etc/profile.d/rbenv.sh

# install ruby-build
RUN mkdir /usr/local/rbenv/plugins
RUN git clone https://github.com/sstephenson/ruby-build.git /usr/local/rbenv/plugins/ruby-build

ENV RBENV_ROOT /usr/local/rbenv

ENV PATH "$RBENV_ROOT/bin:$RBENV_ROOT/shims:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games"

RUN echo $PATH

# does not work. PATH is set to
# $RBENV_ROOT/shims:$RBENV_ROOT/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin