docker use $(nproc)

[Borda]

What does this PR do?

it shall speed up the docker building

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[codecov]

Codecov Report

Merging #7606 (b6bc0b6) into master (20f6337) will decrease coverage by 5%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #7606    +/-   ##
=======================================
- Coverage      92%     88%    -5%     
=======================================
  Files         197     197            
  Lines       12878   12878            
=======================================
- Hits        11899   11312   -587     
- Misses        979    1566   +587