Support running CVAT with an external database via Docker Compose

[SpecLad]

Motivation and context

This would help users who don't want or need the complexity of Kubernetes, but would still like to use an external database.

With Docker Compose, you can initialize a secret from an environment variable, but you can't load a secret into an environment variable. So in order to be able to read the DB password from a secret, I had to introduce a new CVAT_POSTGRES_PASSWORD_FILE variable.

How has this been tested?

Manual testing.

Checklist

• I submit my changes into the develop branch
• I have created a changelog fragment
• I have updated the documentation accordingly
• [ ] I have added tests to cover my changes
• [ ] I have linked related issues (see GitHub docs)
• [ ] I have increased versions of npm packages if it is necessary
  (cvat-canvas,
  cvat-core,
  cvat-data and
  cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.

[codecov]

Codecov Report

Merging #7055 (25e4ce6) into develop (c6682ff) will decrease coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #7055      +/-   ##
===========================================
- Coverage    82.33%   82.29%   -0.05%     
===========================================
  Files          361      361              
  Lines        39213    39213              
  Branches      3585     3585              
===========================================
- Hits         32286    32270      -16     
- Misses        6927     6943      +16     

Components	Coverage Δ
cvat-ui	77.27% <ø> (-0.09%)	⬇️
cvat-server	86.90% <ø> (ø)