Dockerfile: set code dir as git safe directory

[crazy-max]

follow-up #3114

https://github.com/moby/swarmkit/actions/runs/4090832788/jobs/7054522808#step:5:12

docker run -t -v swarmkit-cache:/go -v /home/runner/work/swarmkit/swarmkit/:/go/src/github.com/docker/swarmkit moby/swarmkit make coverage
error obtaining VCS status: exit status 128
	Use -buildvcs=false to disable VCS stamping.

Looks to be the same issue as moby/moby#44930.

- What I did

Set code dir as git safe directory in the Dockerfile

- How I did it

- How to test it

- Description for the changelog

[codecov-commenter]

Codecov Report

Merging #3115 (9850df9) into master (30042f3) will increase coverage by 61.81%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master    #3115       +/-   ##
===========================================
+ Coverage        0   61.81%   +61.81%     
===========================================
  Files           0      153      +153     
  Lines           0    31099    +31099     
===========================================
+ Hits            0    19224    +19224     
- Misses          0    10323    +10323     
- Partials        0     1552     +1552     

[thaJeztah]

Any idea why we need to set it here? (I recall I needed it somewhere in the past, but don't recall the details 🤔)

[crazy-max]

@thaJeztah Wonder if this is linked to moby/moby#44929 (comment) so git fails. Will see if making it as safe directory solves the issue.

Edit: Looks good as safe dir.

[thaJeztah]

LGTM, thanks!

[thaJeztah]

Gonna bring this one in; CircleCI failure is unrelated, and we'll be removing it either way