Move Docker build files

[AlekSi]

Description

Just a small cleanup.

Readiness checklist

• I added/updated unit tests (and they pass).
• I added/updated integration/compatibility tests (and they pass).
• I added/updated comments and checked rendering.
• I made spot refactorings.
• I updated user documentation.
• I ran task all, and it passed.
• I ensured that PR title is good enough for the changelog.
• (for maintainers only) I set Reviewers (@FerretDB/core), Milestone (Next), Labels, Project and project's Sprint fields.
• I marked all done items in this checklist.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.54%. Comparing base (64bd8ca) to head (403f558).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4180   +/-   ##
=======================================
  Coverage   74.53%   74.54%           
=======================================
  Files         339      339           
  Lines       22786    22786           
=======================================
+ Hits        16984    16985    +1     
+ Misses       4498     4497    -1     
  Partials     1304     1304           

see 5 files with indirect coverage changes

Flag	Coverage Δ
filter-true	68.09% <ø> (+<0.01%)	⬆️
hana-1	13.92% <ø> (ø)
integration	68.09% <ø> (+<0.01%)	⬆️
mongodb-1	4.89% <ø> (ø)
postgresql-1	46.58% <ø> (-0.08%)	⬇️
postgresql-2	49.47% <ø> (+0.08%)	⬆️
postgresql-3	49.36% <ø> (+0.05%)	⬆️
sqlite-1	45.78% <ø> (ø)
sqlite-2	48.63% <ø> (+0.94%)	⬆️
sqlite-3	48.51% <ø> (+0.02%)	⬆️
unit	32.54% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

[rumyantseva]

Looks good!