fix: set workspace as safe directory

[pascaliske]

Hi @mhausenblas,

It seems that with newer Git versions (which is the case due to the base image update) the following error occurs in a workflow with steps inside containers:

fatal: unsafe repository ('/github/workspace' is owned by someone else)
To add an exception for this directory, call:

	git config --global --add safe.directory /github/workspace

To workaround this issue I've added a git config line inside the Docker entry point script.

For more information see here: actions/checkout#766

[mhausenblas]

That should fix #184

[mhausenblas]

Thanks a bunch for this fix @pascaliske