Xcode 10.1 beta2 is now available! 
To select this Stack you just have to open your app on bitrise.io,
go to the Workflow tab (Workflow Editor), and on the Stack tab select the Xcode 10.1.x
stack option, and your next build will start on the Xcode 10.1 stack.
Note: you can now select separate stacks for separate workflows! Read more at: https://blog.bitrise.io/per-workflow-stack-selection-is-here.
You can find the Stack’s full system report at:
https://github.com/bitrise-io/bitrise.io/blob/master/system_reports/osx-xcode-10.1.x.log.
If you’d like to add additional tools to be pre-installed you can find the instructions on GitHub, for both the Linux and for the macOS stacks.
Information about Stack types & update schedules can be found here: discuss.bitrise.io/t/281.
Happy Building! 