Add improvements

[rszamszur]

Checklist:

• Added tests for changed code where applicable.
• Documentation reflects the changes where applicable.
• Updated the CHANGELOG.md file with your changes.
• My PR is ready to review.

Resolves: #57
Resolves: #59

Description of the changes being introduced by the pull request:

• Add minor ShellUtils unit test case improvement.
• Allow overriding poetry version for make install target via env variable: $POETRY_VERSION
• Add nix shell config for local development environment

[github-actions]

🚀 Deployed on https://624b639d3571e62819468c09--fastapi-mvc.netlify.app

[codecov-commenter]

Codecov Report

Merging #58 (bb2a9a1) into master (467f29b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #58   +/-   ##
=======================================
  Coverage   99.49%   99.49%           
=======================================
  Files          34       34           
  Lines         591      591           
=======================================
  Hits          588      588           
  Misses          3        3           

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