git: Set /repo as safe directory

* https://stackoverflow.com/questions/69470009/git-diff-cached-unknown-option-cached
* Fixes
  git.exc.GitCommandError: Cmd('git') failed due to: exit code(129)
    cmdline: git diff --cached --abbrev=40 --full-index --raw
    stderr: 'error: unknown option `cached'
  usage: git diff --no-index [<options>] <path> <path>

Closes #24

Dockerfile

@@ -17,6 +17,8 @@ RUN . /etc/profile.d/bsenv.sh \
 	&& echo y | sdkmanager "build-tools;32.0.0" \
 	&& apt-get clean && rm -rf /var/lib/apt/lists/*
 
+RUN git config --global safe.directory /repo
+
 VOLUME ["/repo"]
 WORKDIR /repo