[Docker CI] Clean info/error messages when building the docker image

See https://gitlab.com/tezos/tezos/-/jobs/379603562 like

425        ocaml src/lib_version/generated_git_info.ml
426 fatal: not a git repository (or any of the parent directories): .git
427 fatal: not a git repository (or any of the parent directories): .git
428        ocaml src/lib_version/generated_git_info.ml
429 fatal: not a git repository (or any of the parent directories): .git
430 fatal: not a git repository (or any of the parent directories): .git
431        ocaml src/lib_version/generated_git_info.ml
432 fatal: not a git repository (or any of the parent directories): .git
433 fatal: not a git repository (or any of the parent directories): .git