Commented the cmake file

Dockerfile.cmake

@@ -1,11 +1,19 @@
+# Base the image on the standard C++ stack
 FROM eclipse/cpp_gcc
 
+# Basic system hygiene
 RUN sudo apt-get -y update && \
     sudo apt-get -y upgrade && \
+
+# Now create a directory, get the cmake code and install it
     cd /usr/local/bin && \
     sudo curl -O https://cmake.org/files/v3.8/cmake-3.8.0-Linux-x86_64.tar.gz && \
     sudo tar -xf cmake-3.8.0-Linux-x86_64.tar.gz && \
     sudo rm cmake-3.8.0-Linux-x86_64.tar.gz && \
+
+# Put a symlink to the executable in the PATH
     sudo ln -s /usr/local/bin/cmake-3.8.0-Linux-x86_64/bin/cmake /usr/local/bin/cmake 
 
+# IMPORTANT: The Dockerfile has to end with a non-terminating CMD. This works
+# as well as any....
 CMD tail -f /dev/null