Add note regarding use of `env` and `customParameters`

[tobyspark]

Changes

• Added a clarifying single-sentence note. See discussion in https://github.com/game-ci/docker/issues/229

Checklist

• [x] Read the contribution guide and accept the code of conduct
• [x] Readme (updated or not needed)
• [x] Tests (added, updated or not needed)

[github-actions[bot]]

[GabLeRoux]

Thanks for the contribution, I slightly edited the quote to add Note that ... instead of Note .... I also fixed linter 👍