dockerfile: silence git warning about unsafe owner of directory

as seen here:

Using docker image sha256:1316da8c57224231ba4722a709b938dc11fc3848e7a0c26f2cf54c6b937459d8 for alpinelinux/abuild-ci with digest alpinelinux/abuild-ci@sha256:8e4b9e0f0d879d137bfd19eddd6f85e61db655d367e84eceeecf0c6efc1549ed ...
fatal: unsafe repository ('/builds/ptrcnull/abuild' is owned by someone else)
To add an exception for this directory, call:
	git config --global --add safe.directory /builds/ptrcnull/abuild