Docker: Shallow clone repo

[SuperSandro2000]

This reduces the final repo size by 20MB by not cloning the entire git history. In the future the saving should get bigger.

docker images | grep weserv
weserver-after                              latest              28890e4580b6        30 seconds ago      1.2GB
weserver-before                             latest              e205949007a7        2 minutes ago       1.22GB

[codecov]

Codecov Report

Merging #230 into 5.x will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               5.x      #230   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           57        57           
  Lines         1598      1598           
=========================================
  Hits          1598      1598           

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[kleisauke]

Thanks! It's always a pleasure to see the Dockerfile being improved.