add paragraph on changing the Dockerfile

README.md

@@ -83,7 +83,7 @@ Notice that some changes can affect the overall working of the container
 uppercase letters, `.` being replaced by `_` (e.g. `https.key_store_path` ->
 `HTTPS_KEY_STORE_PATH`).
 
-### Use with Redis
+### Example: Use with Redis
 
 Cantaloupe supports [Redis](https://redis.io) as an external key-value store for caching.
 
@@ -102,3 +102,13 @@ docker run -d --name cantaloupe \
   -v<path_to_host_image_folder>:/home/cantaloupe/images \
   cantaloupe:4.x
 ```
+
+## Hacking Docker Images
+
+Because of the relatively wide variety of possible Docker images, the `Dockerimage` as basis for the build is generated dynamically during the CI/CD process. A small executable `generate-dockerfile` in the root directory helps to produce the target Dockerfile out of a [template](./Dockerfile.template).
+
+You have different options to hack the Docker image generation:
+
+* Call the executable with custom parameters. See [`.gitlab-ci.yml`](./gitlab-ci.yml) for a few ideas.
+* Change the template: The template uses a few simple statements from the [tera template language](https://tera.netlify.app/).
+* Adapt the executable directly. You can find the source code in the directory [`generate-dockerfile-src`](./generate-dockerfile-src).