Update composer Docker tag to v2.7.9

This MR contains the following updates:

Configuration

Schedule: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

Automerge: Disabled by config. Please merge this manually once you are satisfied.

Rebasing: Whenever MR becomes conflicted, or you tick the rebase/retry checkbox.

Ignore: Close this MR and you won't be reminded about this update again.

• If you want to rebase/retry this MR, check this box

This MR has been generated by Renovate Bot.