How do I find out what version of SQL Server for Linux I have?
Using dpkg

dpkg -l | grep -i mssql-server
ii  mssql-server                              14.0.3008.27-1                              amd64        Microsoft SQL Server Relational Database Engine


Using PAL_PROGRAM_INFO=1

You can set the undocumented environmental variable PAL_PROGRAM_INFO for a lot of information,

PAL_PROGRAM_INFO=1 ./sqlservr 
sqlservr
  Version 14.0.3008.27
  Build ID 5fb3474a5f63ad2f4b7eddadad44a086839721f18a66c5fb5d7cfcce25c0f539
  Build Type release
  Git Version 693be7c
  Built at Sat Nov 18 04:54:59 GMT 2017

PAL
  Build ID a984d7631c4f1eb55ae1c77cfb51a09b905528d4e05e728a6343af04e744d96f
  Build Type release
  Git Version 2fe1654
  Built at Sat Nov 18 04:54:06 GMT 2017

Packages
  system.sfp                    6.2.9200.2,2fe1654c9,
  system.common.sfp             10.0.15063.540
  system.certificates.sfp       6.2.9200.2,2fe1654c9,
  system.netfx.sfp              4.6.1590.0
  secforwarderxplat.sfp         14.0.3008.27
  sqlservr.sfp                  14.0.3008.27