Add comments to .env about applicability of the environment variables

[samdark]

Q	A
Is bugfix?	❌
New feature?	❌
Breaks BC?	❌
Fixed issues	-

[Copilot]

Pull request overview

Adds clarifying header comments to Docker environment variable files to document where variables are expected to be visible (Compose interpolation vs container/app runtime).

Changes:

• Add header comments to docker/dev/.env, docker/test/.env, and docker/prod/.env about variables being available in the app/container rather than compose.yml.
• Add a header comment to docker/.env describing variable applicability.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
docker/test/.env	Adds header comment clarifying intended scope of test env vars.
docker/prod/.env	Adds header comment clarifying intended scope of prod env vars.
docker/dev/.env	Adds header comment clarifying intended scope of dev env vars.
docker/.env	Adds header comment about variable scope (currently misleading vs actual Compose behavior).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.96%. Comparing base (71d3a0f) to head (cef41ca).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #453   +/-   ##
=========================================
  Coverage     76.96%   76.96%           
  Complexity       29       29           
=========================================
  Files            11       11           
  Lines           178      178           
=========================================
  Hits            137      137           
  Misses           41       41           

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[vjik]

add changelog

[samdark]

@vjik that's pure docs. Nothing in the code changed, nor in user-faced behavior.