Support reusable postgres with custom DB name

[orlangure]

Fixes #774

[codecov-commenter]

Codecov Report

Base: 86.11% // Head: 86.23% // Increases project coverage by +0.12% 🎉

Coverage data is based on head (38feb43) compared to base (0cb0065).
Patch coverage: 76.47% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #785      +/-   ##
==========================================
+ Coverage   86.11%   86.23%   +0.12%     
==========================================
  Files          49       49              
  Lines        2319     2325       +6     
==========================================
+ Hits         1997     2005       +8     
+ Misses        167      166       -1     
+ Partials      155      154       -1     

Impacted Files	Coverage Δ
preset/postgres/preset.go	87.80% <76.47%> (+0.96%)	⬆️
docker.go	92.20% <0.00%> (+0.86%)	⬆️

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