[docs] Added description how to run task inside a docker container

Author: Michal Tichák

README.md

@@ -191,7 +191,7 @@ There are two ways of interacting with AliECS:
       * [Hashing to aggregate](/docs/metrics.md#hashing-to-aggregate)
       * [Sampling reservoir](/docs/metrics.md#sampling-reservoir)
   * [OCC API debugging with grpcc](/docs/using_grpcc_occ.md#occ-api-debugging-with-grpcc)
-
+  * [Running tasks inside docker](/docs/running_docker.md#running-a-task-inside-a-docker-container)
 * Resources
   * T. Mrnjavac et. al, [AliECS: A New Experiment Control System for the ALICE Experiment](https://doi.org/10.1051/epjconf/202429502027), CHEP23
 

docs/running_docker.md

@@ -0,0 +1,116 @@
+# Running a Task Inside a Docker Container
+
+> ⚠️ **Warning**
+> This method is **not intended for production use**.
+> It serves only as a **proof of concept** for testing Docker images as part of an existing pipeline.
+>
+> Currently, it has been tested with the `alma9-flp-node` image running the *readout* component.
+
+---
+
+## How To
+
+### 1. Manual Setup
+
+Before running tasks in Docker, ensure that the host machine has Docker installed.
+At the time of writing, Docker must be installed **manually**.
+
+> ⚠️ **Security Note**
+> The `flp` user must be able to run `sudo` **without a password**, because Docker requires root privileges because of inter process communication requirements
+>
+> This setup is **potentially unsafe** for production systems. There exists rootless mode in Podman (alias for Docker at RHEL) which might solve safety
+> issues. However, we were not able to make this work for more than one container because of ipc requirements.
+
+Run the following commands as `root` to add `flp` user to sudoers:
+
+```bash
+usermod -aG wheel flp
+echo '%wheel ALL=(ALL) NOPASSWD: ALL' > /etc/sudoers.d/90-wheel-nopasswd
+```
+
+---
+
+### 2. Modifying ControlWorkflows
+
+To run a task inside a Docker container on the executor, wrap the binary call in a `docker run` command within the [ControlWorkflows](https://github.com/AliceO2Group/ControlWorkflows) repository.
+
+For example, to run readout, modify the `_plain_command` section of [`readout.yaml`](https://github.com/AliceO2Group/ControlWorkflows/blob/master/tasks/readout.yaml) by adding a Docker command.
+
+> 🧩 **Note**
+> You must already have a Docker image that includes the required binary and configuration.
+> (Creating such an image is outside the scope of this document.)
+
+#### Example Command
+
+When running readout, we successfully used the following command inside the `alma9-flp-node` image:
+
+```bash
+sudo /usr/bin/docker run --name readout --replace \
+  --user "$(id -u flp):$(id -u flp)" \
+  --network=host --ipc=host \
+  -e O2_DETECTOR -e O2_PARTITION -e OCC_CONTROL_PORT \
+  -e O2_SYSTEM -e O2_ROLE \
+  gitlab-registry.cern.ch/aliceo2group/dockerfiles/alma9-flp-node:2 \
+  /opt/o2/bin/o2-readout-exe
+```
+
+> 🧩 **Note**
+> We are not claiming that this is the most efficient way how to run this image, just that it works.
+
+#### Environment Variables
+
+To identify all required environment variables:
+
+1. Open the **ECS GUI**.
+2. Go to the **Environment Details** page for the relevant task.
+3. Review the variables defined there — these match those used when running the binary outside Docker.
+
+#### Shared Memory Communication
+
+To enable shared memory communication between processes, add the `--ipc=host` flag when running the container.
+However, doing so requires **elevated privileges**.
+
+While **Podman** can run without root privileges, it pauses other Podman processes for the same user.
+This means commands like `podman ps -a` or starting multiple containers in parallel will not work.
+
+Therefore, you should run containers using the same user as the rest of the pipeline:
+
+```bash
+--user "$(id -u flp):$(id -u flp)"
+```
+
+This ensures shared memory segments are created under the same user context.
+
+---
+
+## Tips and Tricks
+
+* Production systems running RHEL do not install native Docker via:
+
+  ```bash
+  dnf install docker
+  ```
+
+  Instead, they use **Podman**, which emulates Docker’s behavior but may differ in certain aspects.
+
+* To check whether ECS has started a container, run:
+
+  ```bash
+  docker ps -a
+  ```
+
+  This lists all containers that have run (or are currently running) under the current user.
+
+  > ECS typically runs as the `flp` user, so to inspect its containers, switch users first:
+  >
+  > ```bash
+  > su - flp
+  > ```
+
+* To view container logs directly from Docker:
+
+  ```bash
+  docker logs <container-id|name>
+  ```
+
+---