How to access the Log-file in Docker-based tests

I use a Docker based test setting for an R-package.

* checking serialization versions ... OK
* checking whether package ‘breathteststan’ can be installed ... 
Installation failed.
See ‘/builds/dmenne/breathteststan/breathteststan.Rcheck/00install.out’ for details. 

How do I get access to the error log 00install.out? I tried

      when: on_failure
        - /builds/dmenne/gastempt/gastempt.Rcheck/00install.out

but this fails, probably because I have not assigned the volume. Any help with the syntax?