Using dotnet build command, you may specify --runtime flag
-r|–runtime < RUNTIME_IDENTIFIER >
Target runtime to build for. For a list of Runtime Identifiers (RIDs) you can use, see the RID catalog.
RIDs that represent concrete operating systems usually follow this pattern [os].[version]-[arch]
Fo example, to build a project and its dependencies for Ubuntu 16.04 runtime use:
dotnet build --runtime ubuntu.16.04-x64