Add group id option to Dockerfile.

[drodarie]

What is new:

• Add group_id option to Dockerfile
• Remove EPFL proxys for BB5
• Update README.md
• Check provided user ids to prevent similar ones

Still to address:

• The line 22 in utils.sh seems to be doing nothing since the functions in declare do not exist:

  su "$user_name" -c "$(declare -f configure_user add_aliases improve_prompt configure_jupyter) &&  configure_user"