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

cat Dockerfile.badruby | docker build -t deepak/badruby -

...

Step 11 : ENV RBENV_ROOT /usr/local/rbenv
---> Running in 57293936038d
---> f48435fd1f71
Step 12 : 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"
---> Running in bac27d8ed4be
---> 0e088f91da43
Step 13 : RUN echo $PATH
---> Running in 80aefab5e4ea
---> 01727253a304

docker logs 80aefab5e4ea
"$RBENV_ROOT/bin:$RBENV_ROOT/shims:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games"

docker run deepak/badruby /bin/sh -c "echo $PATH"
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games