Modified build process to use a built wheel instead of poetry

[banginji]

The Docker image is built as a multi-stage build:

• Build stage uses Poetry plus poetry-plugin-export to:
  • export runtime dependencies from poetry.lock to requirements.txt
  • build a wheel for this project
• Runtime stage installs using pip from:
  • exported requirements.txt
  • built wheel

This keeps Poetry out of the runtime/install path while retaining lockfile-pinned dependency resolution.

This fixes #40