Those aren't all of the instructions for apt-get. The full instructions are on https://docs.microsoft.com/en-us/sql/linux/quickstart-instal... and appear to consist of:

  curl https://packages.microsoft.com/keys/microsoft.asc | sudo apt-key add -
  sudo add-apt-repository "$(curl https://packages.microsoft.com/config/ubuntu/16.04/prod.list)"
  sudo apt-get update
  sudo apt-get install -y mssql-server
  sudo /opt/mssql/bin/mssql-conf setup
  sudo apt-get install -y mssql-tools unixodbc-dev
  echo 'export PATH="$PATH:/opt/mssql-tools/bin"' >> ~/.bash_profile
  echo 'export PATH="$PATH:/opt/mssql-tools/bin"' >> ~/.bashrc
  source ~/.bashrc

And at that point, you have a limited period trial license of a release candidate version installed, and so once you want to use it for production, you have to start dealing with license management issues, which could limit your ability to easily set up dev envs, test envs, hot spares, etc.

All of that is on top of the issues of how heavyweight it is due to containing a platform abstraction layer that consists of a very large amount of Windows running in user mode, as discussed by others in this thread.