add git protocol v2 support via SSH on Docker image

[lunny]

partially fix #4002 according @rgiyf 's reference. see https://docs.gitlab.com/ee/administration/git_protocol.html

[codecov-io]

Codecov Report

Merging #5520 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5520      +/-   ##
==========================================
- Coverage   37.61%   37.58%   -0.03%     
==========================================
  Files         317      318       +1     
  Lines       46835    46928      +93     
==========================================
+ Hits        17619    17640      +21     
- Misses      26712    26776      +64     
- Partials     2504     2512       +8

Impacted Files	Coverage Δ
models/review.go	59.77% <0%> (-4.3%)	⬇️
models/branches.go	43.33% <0%> (-4%)	⬇️
routers/repo/issue.go	37.28% <0%> (-0.28%)	⬇️
models/org_team.go	49.3% <0%> (-0.1%)	⬇️
routers/repo/setting_protected_branch.go	40.23% <0%> (-0.04%)	⬇️
models/migrations/migrations.go	2.61% <0%> (ø)	⬆️
modules/auth/repo_form.go	39% <0%> (ø)	⬆️
models/migrations/v74.go	0% <0%> (ø)
models/pull.go	51.42% <0%> (+0.15%)	⬆️
models/repo_list.go	64.55% <0%> (+1.26%)	⬆️

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[probwebdev]

That's a bad idea. Gitea have a custom sshd config for docker image:
https://github.com/go-gitea/gitea/blob/30ce1b183e9a1f8795be0c2a29a1dbc24a6a3eeb/docker/etc/ssh/sshd_config

[lunny]

@rgiyf right, updated.