Fix Linux MSSQL Customize Example

[Warfront1]

The Linux MSSQL Example had logic errors, as well as breaking (non-executing) code. In addition the README was updated to be platform agnostic (previously it did NOT work on windows). The gitattributes was added to ease with windows development and line endings on bash/shell scripts.

[ghost]

All CLA requirements met.

[a-skua]

Hey, I think not to work this. https://github.com/microsoft/mssql-docker/blob/e50d08e4fe8f2ba276d03e7f9025f9a013d871b4/linux/preview/examples/mssql-customize/configure-db.sh#L13

- i=$i+1
+ i=$(expr $i + 1)

[LuchiLucs]

@Warfront1 did you solve your issue? Would you mind pointing out the logical problems and your solution to them? Thanks