feat: added official glci Docker image (#1)

[mdubourg001]

No description provided.

[carlfischerjba]

Thanks @mdubourg001 for adding this and @unacceptable for proposing it.

Unfortunately, this isn't working for me. When I use the glci container to run a pipeline, the job runs in directory /4e436a8 (corresponds to my latest commit) but the source code isn't available and it can't proceed. It sees only an empty directory called $CI_PROJECT_DIR (that's the name of the directory, dollar symbol included). The glci container itself has access to my source code in /app but this isn't propagated to the container that runs the job. I haven't tested glci outside of Docker so don't know if the same issue occurs.