Dockerfile for docs copy proper files into image

[techknowlogick]

fixes docker build from: #2874

drone-docker plugin uses the base dir as context even when referring to other Dockerfiles in different dir

[codecov-io]

Codecov Report

Merging #2989 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2989      +/-   ##
==========================================
+ Coverage   32.48%   32.51%   +0.02%     
==========================================
  Files         267      267              
  Lines       39271    39271              
==========================================
+ Hits        12759    12769      +10     
+ Misses      24720    24714       -6     
+ Partials     1792     1788       -4

Impacted Files	Coverage Δ
models/repo.go	38% <0%> (+0.18%)	⬆️
models/repo_indexer.go	49% <0%> (+3.46%)	⬆️

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[lunny]

I think maybe add a route /docs to the folder and link Help to self /docs.

[techknowlogick]

Thanks for feedback @lunny

For this PR I'm just trying to keep it as simple as I can to fix this build when on master branch: https://drone.gitea.io/go-gitea/gitea/2031/17

[lunny]

@techknowlogick That's OK. That could be done by future PRs.

[lafriks]

LGTM

[lunny]

LGTM

[lafriks]

@techknowlogick still failing :(