Fix docker build (kripken#360)

* Fix and document Docker build

* Fix Docker build in case the targets are not present

README.markdown

@@ -156,6 +156,19 @@ On windows, you can build using cmake's
 Note that if you have not installed emscripten via the emsdk, you can configure
 its location with `-DEMSCRIPTEN_ROOT`.
 
+### Building using Docker
+
+ammo.js can also be built with [Docker](https://www.docker.com).
+This offers many advantages (keeping its native environment clean, portability, etc.).
+To do this, you just have to install Docker and run:
+
+  ```bash
+  $ docker-compose build  # to create the Docker image
+  $ docker-compose up     # to create the Docker container and build ammo.js
+  ```
+
+If you want to add arguments to cmake, you have to edit the `docker-compose.yml` file. 
+
 Reducing Build Size
 -------------------
 

docker-compose.yml

@@ -5,4 +5,4 @@ services:
     build: .
     volumes:
       - .:/code
-    command: sh build.sh 
+    command: bash -c "rm -f builds/ammo.* && cmake -B builds && cmake --build builds"