add a length constraint on 'docker_name'
When the docker_name is too long, the jobs/sandboxes fail to start with the following error:
docker.errors.APIError: 500 Server Error: Internal Server Error ("b'OCI runtime create failed: container_linux.go:345: starting container process caused "process_linux.go:430: container init caused \\"sethostname: invalid argument\\"": unknown'"))
https://support.inria.fr/Ticket/Display.html?id=144624
The hostname is limited to 64 characters, and we need a margin because a prefix is prepended to the name of the container (ex : 'prod-sandbox-', 'qualif-sandbox'-, 'prod-jobXXX').
-> we should limit the length of 'docker_name' to 30 chars
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