+ docker custom configs

docs/10-prerequisities/30-docker/60-custom-configs.md

@@ -6,4 +6,72 @@ description: Custom configs for containers
 
 # Custom configs for containers
 
-TODO
+## Problem definition
+
+A users has a **docker image** from which his software can be started.
+The image contains pre-compiled binaries with a **default configuration**.
+However, in many cases (robotics applications), the user's software needs to be started with **different configuration** each time.
+Configuration can be facilited by several mechanisms:
+
+* config files,
+* ROS launch files,
+* shell scripts,
+* additional files loaded in runtime.
+
+These **custom configuration files**, which are handed to the software in runtime, need to be exposed to the **container** runing from the **user's docker image**.
+
+## Our solution
+
+We pack the configurations (**shared data**) in a dedicated transport image which is easily delivered to the robot.
+Then, **during runtime**, this image spawns a container in which the data is coppied to a **shared docker volume**.
+Then, the user's containers are started, all mounting the same **shared docker volume** and accessing the configuration data.
+
+![](./fig/containers_shared_data.png)
+
+## Packing shared data to a transport image
+
+The following minimalistic dockerfile coppies the `shared_data` subfolder in the **transport docker image**:
+
+```dockerfile
+FROM alpine
+
+COPY shared_data/ /etc/docker/shared_data/
+
+CMD ["sh"]
+```
+
+Build the image using the following command.
+Select the appropriate `tag` and `platform`.
+
+```bash
+docker build build . --file Dockerfile --tag <shared_data_image_tag> --platform=linux/<my_platform>
+```
+
+## Unloading the shared data from the transport image
+
+The following compose session shows the process of unloading the shared data.
+The `user_program` can then access the files in the shared docker volume.
+
+```yaml
+volumes:
+
+  shared_data:
+
+services:
+
+  # will copy session-specific data shared between containers from the shared_data container to a shared volume
+  copy_shared_data:
+    image: <shared_data_image_tag>
+    volumes:
+      - shared_data:/tmp/docker/shared_data:consistent
+    tty: true
+    command: sh -c "rm -rvf /tmp/docker/shared_data/*; mkdir -pv /tmp/docker/shared_data; cp -rv /etc/docker/shared_data/* /tmp/docker/shared_data/"
+
+  user_program:
+    image: <users_base_image>
+    depends_on:
+      - copy_shared_data
+    volumes:
+      - shared_data:/etc/docker/shared_data:consistent
+    command: my_process /etc/docker/shared_data/config_file.txt
+```