Moving a SQL Server DB from Windows to Linux fails
In the first release of SQL Server on Linux, I saw that SQL Server translates paths to related terms in windows, for example, root '/' will be C:. 
so if this logic exists, you may try to make it something like: 


  FROM  DISK = N'C:\var\opt\mssql\backup_file_name.bak' WITH  FILE = 1


but I think it's different now as mentioned here
maybe you should first check the version of your SQL Server on Linux and then check the steps on that article.