Setting custom cache directory in docker image leads to absolute directory error

[Int-Circuit]

What version of Hugo are you using (hugo version)?

hugo-docker:latest

Does this issue reproduce with the latest release?

Yes.

Steps to reproduce the bug.

• Create .gitlab.ci.yaml with job to deploy/test hugo website
• Specify custom cache dir using HUGO_CACHEDIR: cache/test as global variable (required as Gitlab CI runner cannot write to /cache/ due to permissions errors)
• Get the following error:

Error: failed to create config from result: failed to decode "caches": "cache/test must resolve to an absolute directory

Mitigations to this problem

Please document the configuration variable of the Docker image.