In a Dockerfile, How to update PATH environment variable?

I have a dockerfile that download and builds GTK from source, but the following line is not updating my image's environment variable: ``` RUN PATH="/opt/gtk/bin:$PATH" RUN export PATH ``` I read that that I should be using ENV to set environment values, but the following instruction doesn't seem to work either: `ENV PATH /opt/gtk/bin:$PATH` This is my entire Dockerfile: ``` 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 ```