Update ci_faq.md

documentation/ci_faq.md

@@ -28,6 +28,11 @@ condition.
      thing is to fix the test so that we continue to have good regression
      test coverage over our code.
 
+## My build failed due to unavailable software. How do I install things on CI?
+Each build is run within a docker image. Add whatever software you need on the
+`sudo apt install ...` line inside of the `.gitlab-ci.yml` file in the root
+directory.
+
 ## How does CI work?
 When a commit is added to a branch in our repository, a notification is sent out
 to our gitlab-ci-runner, which instructs it to checkout the updated branch and