Tweak docs re in-docker usage

docs/usage/dind.md

@@ -1,13 +1,18 @@
 # Docker in Docker
 
-Since TestContainers version 1.1.8, you can run your tests inside a container.
-It's very useful for different CI scenarios like running everything in containers on Jenkins.
+> Since 1.1.8
+
+Testcontainers itself can be used from inside a container.
+This is very useful for different CI scenarios like running everything in containers on Jenkins, or Docker-based CI tools such as Drone.
 
 TestContainers will automatically detect if it's inside a container and instead of "localhost" will use the default gateway's IP.
 
-However, an additional configuration is required if you use [volume mapping](options.md#volume-mapping).
+However, additional configuration is required if you use [volume mapping](options.md#volume-mapping). The following points need to be considered:
+
+* The docker socket must be available via a volume mount
+* The 'local' source code directory must be volume mounted *at the same path* inside the container that Testcontainers runs in, so that Testcontainers is able to set up the correct volume mounts for the containers it spawns.
 
-## Docker-only
+## Docker-only example
 If you run the tests with just `docker run ...` then make sure you add `-v $PWD:$PWD -w $PWD -v /var/run/docker.sock:/var/run/docker.sock` to the command, so it will look like this:
 ```bash
 $ tree .
@@ -26,7 +31,7 @@ Where:
 * `-w $PWD` will set the current directory to this volume
 * `-v /var/run/docker.sock:/var/run/docker.sock` will map the Docker socket
 
-## Docker Compose
+## Docker Compose example
 The same can be achived with Docker Compose:
 ```yaml
 tests: