README: add docker test environment usage info.

README.md

@@ -449,6 +449,44 @@ make stats-lisp^IMPL
 make stats-lisp^js
 ```
 
+## Docker test environment
+
+There is a Dockerfile included in the `tests/docker` directory that
+builds a docker image based on Ubuntu Utopic that contains everything
+needed to run tests against all the implementations (except for MATLAB
+which is proprietary/licensed).
+
+Build the the docker image using a provided script. WARNING: this will
+likely take over an hour to build from scratch and use more 3 GB of disk:
+```bash
+./tests/docker-build.sh
+```
+
+Launch a docker container from that image built above. This will
+volume mount the mal directory to `/mal` and then give you a bash
+prompt in the container. You can then run individual mal
+implementations and tests:
+```bash
+./tests/docker-run.sh
+```
+
+You can also specify a command to run within the container. For
+example, to run step2 tests for every implementation (except MATLAB):
+```bash
+./tests/docker-run.sh make SKIP_IMPLS="matlab" test^step2
+```
+
+**Notes**:
+* JVM-based language implementations (Java, Clojure, Scala): you will
+  need to run these implementations once manually first before you can
+  run tests because runtime dependencies need to be downloaded to
+  avoid the tests timing out. These dependencies are download to
+  dot-files in the /mal directory so they will persist between runs.
+* Compiled languages: if you host system is different enough from
+  Ubuntu Utopic then you may need to re-compile your compiled
+  languages from within the container to avoid linker version
+  mismatches.
+
 
 ## License