title: In a Dockerfile, How to update PATH environment variable?
body: <p>I have a dockerfile that download and builds GTK from source, but the following line is not updating my image's environment variable:</p>
  
  <pre><code>RUN PATH="/opt/gtk/bin:$PATH"
  RUN export PATH
  </code></pre>
  
  <p>I read that that I should be using ENV to set environment values, but the following instruction doesn't seem to work either:</p>
  
  <p><code>ENV PATH /opt/gtk/bin:$PATH</code></p>
  
  <p>This is my entire Dockerfile:</p>
  
  <pre><code>FROM ubuntu
  RUN apt-get update
  RUN apt-get install -y golang gcc make wget git libxml2-utils libwebkit2gtk-3.0-dev libcairo2 libcairo2-dev libcairo-gobject2 shared-mime-info libgdk-pixbuf2.0-* libglib2-* libatk1.0-* libpango1.0-* xserver-xorg xvfb
  
  # Downloading GTKcd
  RUN wget http://ftp.gnome.org/pub/gnome/sources/gtk+/3.12/gtk+-3.12.2.tar.xz
  RUN tar xf gtk+-3.12.2.tar.xz
  RUN cd gtk+-3.12.2
  
  # Setting environment variables before running configure
  RUN CPPFLAGS="-I/opt/gtk/include"
  RUN LDFLAGS="-L/opt/gtk/lib"
  RUN PKG_CONFIG_PATH="/opt/gtk/lib/pkgconfig"
  RUN export CPPFLAGS LDFLAGS PKG_CONFIG_PATH
  RUN ./configure --prefix=/opt/gtk
  RUN make
  RUN make install
  
  # running ldconfig after make install so that the newly installed libraries are found.
  RUN ldconfig
  
  # Setting the LD_LIBRARY_PATH environment variable so the systems dynamic linker can find the newly installed libraries.
  RUN LD_LIBRARY_PATH="/opt/gtk/lib"
  
  # Updating PATH environment program so that utility binaries installed by the various libraries will be found.
  RUN PATH="/opt/gtk/bin:$PATH"
  RUN export LD_LIBRARY_PATH PATH
  
  # Collecting garbage
  RUN rm -rf gtk+-3.12.2.tar.xz
  
  # creating go code root
  RUN mkdir gocode
  RUN mkdir gocode/src
  RUN mkdir gocode/bin
  RUN mkdir gocode/pkg
  
  # Setting the GOROOT and GOPATH enviornment variables, any commands created are automatically added to PATH
  RUN GOROOT=/usr/lib/go
  RUN GOPATH=/root/gocode
  RUN PATH=$GOPATH/bin:$PATH
  RUN export GOROOT GOPATH PATH
  </code></pre>