add workaround for new git version

fixes:
fatal: detected dubious ownership in repository at '/builds/firmware/site-ffs'

Dockerfile

@@ -26,3 +26,6 @@ RUN apt-get update \
 # gluon build fails when running as root
 RUN adduser --quiet --disabled-login --gecos '' build
 USER build:build
+# workaround for fatal: detected dubious ownership in repository at '/builds/firmware/site-ffs'
+# https://gitlab.com/gitlab-org/gitlab-runner/-/issues/29022
+RUN git config --global --add safe.directory '*'