Support shm_size option for docker

The docker parser (for git repos with workers) should support a new option docker.shm_size.

Code is there : https://gitlab.com/teklia/arkindex/tasks/-/blob/master/arkindex_tasks/import_git/parser.py#L32

The format is . number must be greater than 0. Unit is optional and can be b (bytes), k (kilobytes), m (megabytes), or g (gigabytes)

By default it's None