Add docs on passing SSH through to container

[Xiol]

Wasn't clear how to pass SSH connections through the container, found a
few blog posts which described roughly the same procedure I've
documented here.

Credit should really go to:

• https://blog.sakuragawa.moe/gitea-in-docker-container-and-sharing-ssh-with-host/
• http://www.ateijelo.com/blog/2016/07/09/share-port-22-between-docker-gogs-ssh-and-local-system

Signed-off-by: Dane Elwell dane.elwell@ukfast.co.uk

[codecov-io]

Codecov Report

Merging #5019 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5019      +/-   ##
==========================================
- Coverage   37.81%   37.81%   -0.01%     
==========================================
  Files         322      322              
  Lines       47485    47485              
==========================================
- Hits        17957    17955       -2     
- Misses      26939    26942       +3     
+ Partials     2589     2588       -1

Impacted Files	Coverage Δ
models/unit.go	0% <0%> (-14.29%)	⬇️

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[adelowo]

Fixes #4978

[techknowlogick]

With the plan on removing OpenSSHD from the container, this would no longer work so I'm hesitant to add this to documentation

[lafriks]

When we switch to builtin ssh we can always remove this section but while we have not switched it is still good to have it

[techknowlogick]

@lafriks ok. sounds good.

[lafriks]

Please add note somewhere that this will work only if using opensshd in container and will not work if AuthorizedKeysCommand is used with setting to disable authorized files key generation: https://github.com/go-gitea/gitea/blob/master/custom/conf/app.ini.sample#L157

[techknowlogick]

@lafriks I've updated PR with changes from your feedback, please review