Use unique name for docs docker build step in CI

[techknowlogick]

Otherwise drone may combine (or hide) the logs for the docs image

[lafriks]

LGTM

[codecov-io]

Codecov Report

Merging #3018 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3018      +/-   ##
==========================================
+ Coverage   32.98%   33.01%   +0.02%     
==========================================
  Files         270      270              
  Lines       39534    39534              
==========================================
+ Hits        13042    13052      +10     
+ Misses      24637    24631       -6     
+ Partials     1855     1851       -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]

LGTM