CI: let the docker runner guess the user to clone the git repository with

In the some runners (builder-release.videolan.org, a Videolabs one) we can't run
commands in the git repository:

fatal: detected dubious ownership in repository at '/builds/videolan/vlc'
To add an exception for this directory, call:

	git config --global --add safe.directory /builds/videolan/vlc
cat: /builds/videolan/vlc/extras/package/win32/../../../src/revision.txt: No such file or directory

See https://gitlab.com/gitlab-org/gitlab-runner/-/merge_requests/3538

extras/ci/gitlab-ci.yml

@@ -20,6 +20,7 @@ default:
 
 variables:
     VLC_TEST_TIMEOUT: 60
+    FF_DISABLE_UMASK_FOR_DOCKER_EXECUTOR: "true"
     VLC_WIN64_IMAGE: registry.videolan.org/vlc-debian-win64-posix:20241118101328
     VLC_WIN_LLVM_MSVCRT_IMAGE: registry.videolan.org/vlc-debian-llvm-msvcrt:20250903131032
     VLC_WIN_LLVM_UCRT_IMAGE: registry.videolan.org/vlc-debian-llvm-ucrt:20250903131032