Testbox Caching in Bitbucket Pipelines

So, I was trying to speed up the pipeline (cuz ya only get so many free minutes, ya know), and I read about caching in Bitbucket Pipelines.

To do this, add the following to your test step:

caches:
  - testbox

And add the following cache definition to the bottom of your pipeline config:

definitions:
  caches:
    testbox: testbox

So here's my full setup for testbox with caching:

pipelines:
  default:
    - step:
      deployment: test
      caches:
        - testbox
        script:
          - box install production=true && box server start
          - box testbox run
definitions:
  caches:
    testbox: testbox

Unfortunately, this did not work and totally broke my build! As soon as I ran the pipeline, I got the following error:

ERROR (4.2.0+00071)
Error executing tests:500 Internal Server Error   

Lucee 5.2.9.31 Error (missinginclude) 
Message Page /testbox/system/runners/HTMLRunner.cfm 
[/opt/atlassian/pipelines/agent/build/testbox/system/runners/HTMLRunner.cfm] not found

So it would seem that either I did this wrong... or caching doesn't work quite like I thought in Bitbucket Pipelines.

I would love to get this working - if anyone wants to point out where I'm doing something wrong, go ahead and hit me up on Twitter!