ADD: Hint to function of docker version tags in tutorial

[Wyall]

I wasn't sure about how tags work, maybe this is interesting for people with less knowledge of docker.

But that's entirely up for discussion, basically what the target audience of the guide is.

[codecov-io]

Codecov Report

Merging #4967 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4967      +/-   ##
==========================================
+ Coverage    37.4%   37.41%   +<.01%     
==========================================
  Files         306      306              
  Lines       45334    45334              
==========================================
+ Hits        16957    16960       +3     
+ Misses      25925    25923       -2     
+ Partials     2452     2451       -1

Impacted Files	Coverage Δ
models/repo_indexer.go	52.11% <0%> (+1.27%)	⬆️

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