[clojure-emacs#2144] create a docker image to mimic the travis enviro…

…nment

doc/hacking_on_cider.md

@@ -81,17 +81,17 @@ $ make test-bytecomp
 
 If you prefer to see the full Travis test suite run successfully, the easiest
 way to achieve that is to create your own personal account on
-https://travis-ci.org, and then view your profile details and toggle the switch
-to enables Travis to build your fork of the cider project.
+https://travis-ci.org. View your profile details on the Travis site, and toggle
+the switch to enable builds on your fork of the cider project.
 
-Subsequent pushes to this fork will generate a Travis build you can monitor for
+Subsequent pushes to your fork will generate a Travis build you can monitor for
 success or failure.
 
 #### Simulating the Travis tests locally in Docker
 
 If you prefer not to wait for Travis all the time, or if you need to debug
 something that fails in Travis but does not fail for you on your own machine,
-then you can also run the Travis test suite manually in Docker.
+then you can also run the Travis tests manually in Docker.
 
 You will need to run some scripts to build and launch the Docker image.
 
@@ -132,9 +132,16 @@ You can test byte compilation too
 travis@c3936c77aae0:~/cider$ make elpaclean test-bytecomp
 ```
 
-By default, the docker image pre-installs only the most recent releases of emacs
-25, emacs 26, and a recent snapshot of the emacs git repository. However, the
-`evm` tool is available should you need to install some other specific build.
+When you are done working in docker, just `exit` the bash prompt, and the docker
+container will also exit. Note that `docker/run.sh` runs the container with
+`--rm`, meaning any changes to the docker container are discarded when the
+container exits.
+
+So for example, by default, the docker image pre-installs only the most recent
+releases of emacs 25, emacs 26, and a recent snapshot of the emacs git
+repository. The `evm` tool is available should you need to install some other
+specific build. However additional versions of emacs will be discarded when
+you exit the docker container.
 
 ## Hacking on cider-nrepl