The man page for xcodebuild reads:
xcodebuild
Run xcodebuild from the directory containing your project (i.e. the directory containing the project
For a Workspace if you're using CocoaPods:
xcodebuild -workspace youProject.xcworkspace -scheme yourScheme -sdk iphonesimulator -configuration Release SYMROOT=$(PWD)/build
That will put your .app in ./build/Release-iphonesimulator
./build/Release-iphonesimulator