This folder contains a set of docker config files, usually named [Dockerfile](https://docs.docker.com/engine/reference/builder/), that are used to produce container images which then will serve as environments for running programs.
Docker images are used in Gitlab CI runners to do pipeline tests : Docker images are specified at each `image: foo` line in the `.gitlab-ci.yml` file (at the root of this repository), the image is then used as an environment to run scripts in. In our specific case, we use Docker images to test that our code 'works' but also to check compatibility with specific versions of specific Linux distributions.
### Modifying a Dockerfile
Each docker file is named after the Linux distribution it bundles (ont its latest stable version) using the following pattern `Dockerfile.${Linux_distro_name}` (in lowercase), for example `Dockerfile.debian`.
To learn more about the actual writing of a Dockerfile, a start can be here: https://docs.docker.com/engine/reference/builder/
### Create a Docker image from a config file and upload it
From a given Dockerfile in this repository, one can locally (in his own machine) create a Docker image then upload it the Gitlab repository to be used by the runners for the pipeline.
To create a Docker image, one needs to have Docker installed. On Debian derivatives:
```shell
$ sudo apt install docker.io
$ sudo systemctl enable--now docker
```
Then, one would do the following commands, for the file `Dockerfile.debian`:
Docker tries to upload several files at once when pushing, if kept at its default configuration. And that can lead to aborts if the server is busy or refuses multi-uploads. If it is happening, one can tweak this this behavior for the push to work. In a shell, one can stop the Docker service, change a config file, and then re-start the service:
```shell
$ sudo systemctl stop docker
$ sudo nano /etc/docker/daemon.json
```
Then add the two following lines:
```json
{
"max-concurrent-uploads":1,
"max-concurrent-downloads":1
}
```
Then save and exit (`CTRL+X` then `Y` then `ENTER`) then restart the docker service:
