On CircleCI 2.0 the only supported way to do CodeSigning is to use Fastlane Match as described in the code signing documentation.
Note: For fastlane match to work correctly, you must add setup_circle_ci to before_all in your Fastfile. This ensures that a temporary Fastlane keychain is used.
If you see errors like "Code Signing Error: No profile" or "Provisioning Profile Not Found" it's likely that you don’t have the keychain unlocked.
You can run security find-identity -v -p codesigning from inside the build to list the current status of your code signing credentials.