tell git to trust the repo

Recent versions of git do not like to run as root in repos that are not
owned by root.  This is commonly the situation when a user checks out
a git repo, then runs a docker container with the repo mounted in it.

The version un debian unstable (1:2.38.1-1) has this issue.  The version
in bullseye (1:2.30.2-1) does not have this issue.

git-deborig gives this unhelpful error message:

    $ git deborig HEAD
    pwd doesn't look like a git repository ..

Regular git gives this more useful error message:

    $ git status
    fatal: detected dubious ownership in repository at '/data/home/seb/action-debian-package'
    To add an exception for this directory, call:

            git config --global --add safe.directory /data/home/seb/action-debian-package

This commit fixes the issue by running the recommended command inside
the container, before trying to access the git repo.

Fixes #62.

main.js

@@ -155,6 +155,15 @@ async function main() {
         ])
         core.endGroup()
 
+        core.startGroup("Trust this git repo")
+        await exec.exec("docker", [
+            "exec",
+            container,
+            "bash", "-c",
+            "git config --global --add safe.directory ${PWD}"
+        ])
+        core.endGroup()
+
         if (imageTag != "trusty") {
             core.startGroup("Install build dependencies")
             await exec.exec("docker", [