Add docker build example to README

Now that we use build arguments, people might like an example command.

README.md

@@ -7,3 +7,12 @@ This [demo video](https://youtu.be/DuObpGNpDhw) quickly demonstrates the viewer'
 
 The project [homepage](https://bioinformatics.mdanderson.org/main/NG-CHM-V2:Overview)
 includes additional documentation, introductory videos, and tutorials. 
+
+To build image:
+
+```bash
+docker build \
+       --build-arg="GIT_COMMIT=$(git rev-parse --short HEAD)" \
+       --build-arg="GIT_LATEST_TAG=$(git describe --tags --abbrev=0)" \
+       -t ngchm:latest .
+```