How do you view a log created during gitlab-runner exec?

I am testing a GitLab CI pipeline with gitlab-runner exec. During a script, Boost ran into an error, and it created a log file. I want to view this log file, but I do not know how to.

.gitlab-ci.yml in project directory:

image: alpine


 - apk add cmake
 - cd include/boost
 - sh

I test this on my machine with:

sudo gitlab-runner exec docker build --timeout 3600

The last several lines of the output:

Building Boost.Build engine with toolset ... 
Failed to build Boost.Build build engine
Consult 'bootstrap.log' for more details
ERROR: Job failed: exit code 1
FATAL: exit code 1    

bootstrap.log is what I would like to view.

Appending - cat bootstrap.log to .gitlab-ci.yml does not output the file contents because the runner exits before this line. I tried looking though past containers with sudo docker ps -a, but this does not show the one that GitLab Runner used. How can I open bootstrap.log?