Document the XDG_CACHE_HOME environment variable

[paternal]

Page https://www.getlektor.com/docs/deployment/travisci/ describes how to cache the lektor directory, but the same method cannot be used with Gitlab pages. This pull request explains an alternative method which works with Gitlab.

[dairiki]

You might want to note that setting XDG_CACHE_HOME will also affect the location of caches generated by other programs. E.g. the settings in your example .gitlab-ci.yml will also result in the pip wheel cache being cached. (In general, that’s probably a good thing, but it’s good to be aware that that’s happening.)

FWIW, to avoid surprises, my personal preference is to explicitly call out which caches are being cached. e.g.

variables:
    XDG_CACHE_HOME: "$CI_PROJECT_DIR/_cache"
cache:
    paths:
      - _cache/pip
      - _cache/lektor

[paternal]

@dairiki I agree :) 3220662

[dairiki]

Merged. Thank you, @paternal!