Docker usage focus on using bats isolated in other project

Focus on how to use bats in another project.

Document how one loads libraries, solves bats-core#707.

docs/source/docker-usage.md

@@ -2,11 +2,29 @@
 
 - [Docker Usage Guide](#docker-usage-guide)
   * [Basic Usage](#basic-usage)
+  * [Basic Usage for bats project](#basic-usage-for-bats-project)
   * [Docker Gotchas](#docker-gotchas)
   * [Extending from the base image](#extending-from-the-base-image)
 
 ## Basic Usage
 
+For test suites that are intended to run in isolation from the bats project, you can mount the test directory and run the [official bats docker image](https://hub.docker.com/r/bats/bats):
+
+```bash
+$ docker run -it -v "$PWD:/code" bats/bats:latest /code/test
+```
+
+Docker image includes libaries like [bats-support](https://github.com/bats-core/bats-support) and [bats-assert](https://github.com/bats-core/bats-assert), which can be loaded in `setup` like this:
+
+```bash
+setup() {
+    bats_load_library bats-support
+    bats_load_library bats-assert
+}
+```
+
+## Basic Usage for Bats Project
+
 To build and run `bats`' own tests:
 ```bash
 $ git clone https://github.com/bats-core/bats-core.git
@@ -29,12 +47,6 @@ $ docker run -it -v "$PWD:/opt/bats" bats/bats:latest /opt/bats/test
 ```
 This runs the `test/` directory from the bats-core repository inside the bats Docker container.
 
-For test suites that are intended to run in isolation from the project (i.e. the tests do not depend on project files outside of the test directory), you can mount the test directory by itself and execute the tests like so:
-
-```bash
-$ docker run -it -v "$PWD:/code" bats/bats:latest /code/test
-```
-
 ## Docker Gotchas
 
 Relying on functionality provided by your environment (ssh keys or agent, installed binaries, fixtures outside the mounted test directory) will fail when running inside Docker.