MSBuild Deploy SQL Database Project .dbproj - login fails

I have managed to get TeamCity building a .dbproj file using MSBuild as the build runner. The problem is it fails when it trys to login to the DB. I am using SQL Server Auth. i.e. I have created a builduser account and given it the required permissions. Indeed, if I do 'Build -> Deploy Solution' it works a treat. So why does it fail inTeamCity?

The error is "error Deploy01234: Could not connect to myDBserver : Login failed for user 'builduser' "

Thanks