Added build instructions to README.md

README.md

@@ -3,3 +3,10 @@
 A simple docker image I used to teach me the basics of docker images and how to build them.
 
 
+## Build the container
+
+To build the container to an image called "cmatrix-demo", run
+
+```sh
+docker build -t cmatrix-demo .
+```