Change sa password for SQL Server on a GNU/Linux machine
What OS are you using?

To get a fresh installation you have to remove the data directory, after removing the mssql-server package:


  sudo yum remove mssql-server 
  OR 
  sudo apt-get remove mssql-server 


And then:


  sudo rm -rf /var/opt/mssql


This will delete all your user databases, but will also remove the system databases, which will allow you to reinstall and, hence, re-specify the SA password upon reinstall. 

Let me know if this works.