In many instances for CI machines you will need to provide Code Signing provisioning profiles as well as code signing certifications. Typically, you would have to search on the Apple Developer Portal and do a lot of steps to generate these files. Thankfully, Bitrise has created a special tool to make your life a little easier.
Just paste this code in your terminal and follow the steps.
bash -l -c "$(curl -sfL https://raw.githubusercontent.com/bitrise-tools/codesigndoc/master/_scripts/install_wrap-xcode.sh)"