fix(docker): add git safe dir to data dir

[janderssonse]

Fixes the requirement that: Newer versions of git needs the 'git config --global --add safe.directory /data' setting to work in a container.

Here is an example to verify both the problem and the fix:

Problem:

1. .Build an reuse image from latest main code, official branch:

docker build -f docker/Dockerfile-debian -t reusetest_default .

2). Run the code in reuse compliant project, I used my script project (https://github.com/janderssonse/janderscripts)

docker run --rm --volume $(pwd):/data reusetest_default lint

Result: Fails respecting .gitignore etc as the git config will fail and show warning '---no VCS'' etc.

Fix:
3). Now try the same dockerfile but with the added fix in this pr:

docker build -f docker/Dockerfile-debian -t reusetest_with_git_add_data_safe .

4). Run on same example repo as before (https://github.com/janderssonse/janderscripts)

docker run --rm --volume $(pwd):/data reusetest_with_git_add_data_safe lint

Result: success - git (vcs) accepted, .gitignore respected etc.

[linozen]

Many thanks @janderssonse for bringing this issue up and providing a fix right away. I tested this locally and all seems to work as expected again using your fix.