Added ENV to .env.sample

[MichaelMallaburn]

Description

Fixes #613: Adds the ENV environment variable to .env.sample

Type of change

Please delete options accordingly to the description.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Suggested Checklist

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• My changes generate no new warnings.
• I have commented my code, particularly in hard-to-understand areas.
• I have passed on my local host device. (see further details at the CONTRIBUTING document)
• Make sure your branch is up-to-date with main branch. See CONTRIBUTING for a general example to syncronise your branch with the main branch.
• I have requested review to this PR.
• I have addressed and marked as resolved all the review comments in my PR.
• Finally, I have selected squash and merge

[milanmlft]

Thanks! Though I wouldn't set it to prod by default 🙂

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.69%. Comparing base (8ca52a9) to head (cad0fe6).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #614      +/-   ##
==========================================
- Coverage   87.89%   85.69%   -2.21%     
==========================================
  Files          76       72       -4     
  Lines        3619     3307     -312     
==========================================
- Hits         3181     2834     -347     
- Misses        438      473      +35     

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