Update README.md

Adding instructions for using the pre-built docker image.

README.md

@@ -2,5 +2,14 @@
 
 
 ## Build/Run Docker image for Molssi Python Best practices workshop 
-  1. `docker build -t molecool_env .`
-  2.  `docker run -it molecool_env`
+The dockerfile can be used to build a docker image. Clone this repository and navigate using the command line (a terminal) to the root folder of the repository.
+Run the following command to build the docker image:
+`docker build -t python-best-practices-container .`
+After it has been built, you can run the resulting image as an interactive terminal using
+`docker run -it python-best-practices-container`
+
+The repository also has a pre-built image that can be pulled to your local computer using Docker.
+The container can be found in the packages section of this repository, or pulled using:
+`docker pull ghcr.io/molssi-education/python-best-practices-container:latest`
+Once the image is stored locally, you can the following command to run the image as if you had built it.
+`docker run -it ghcr.io/molssi-education/python-best-practices-container:latest`