Merge pull request #438 from Unity-Technologies/fix/dockerdocs

[docs] Adding information on saving and quitting docker container

docs/Using-Docker.md

 
 Run the Docker container by calling the following command at the top-level of the repository:
 ```
-docker run --mount type=bind,source="$(pwd)"/unity-volume,target=/unity-volume \
-     <image-name>:latest <environment-name> \
-     --docker-target-name=unity-volume \
-     --train --run-id=<run-id>
+docker run --name <container-name> \
+           --mount type=bind,source="$(pwd)"/unity-volume,target=/unity-volume \
+           <image-name>:latest <environment-name> \
+           --docker-target-name=unity-volume \
+           --train --run-id=<run-id>
+- `<container-name>` is used to identify the container (in case you want to interrupt and terminate it). This is optional and Docker will set a random string if this is not set. _Note that this must be unique for every run of a Docker Image_
 - `<image-name>` and `<environment-name>`: References the image and environment names, respectively.
 - `source`: Reference to the path in your host OS where you will store the Unity executable. 
 - `target`: Tells Docker to mount the `source` path as a disk with this name. 
 For the `3DBall` environment, for example this would be:
 
 ```
-docker run --mount type=bind,source="$(pwd)"/unity-volume,target=/unity-volume \
-     balance.ball.v0.1:latest 3Dball \
-     --docker-target-name=unity-volume \
-     --train --run-id=3dball_first_trial
+docker run --name 3DBallContainer.first.trial \
+           --mount type=bind,source="$(pwd)"/unity-volume,target=/unity-volume \
+           balance.ball.v0.1:latest 3Dball \
+           --docker-target-name=unity-volume \
+           --train --run-id=3dball_first_trial
+
+
+### Stopping Container and Saving State
+
+
+If you are satisfied with the progress being made by the algorithm in training, you can stop the docker container, while saving state using the following :
+
+```
+docker kill --signal=SIGINT <container-name>
+```
+
+`<container-name>` is the name of the container that you set during the `docker run`. If you didn't set the container name there, you can find the automatically generated identifier by running `docker container ls`.