fix: Add input for overriding poetry version

[tiagovrtr]

Adds new input poetry_version in order to override the latest version during the container build.
Adds entrypoint script to reinstall poetry at runtime if this input is set
Sets the default version to the latest v1 version of poetry in order to fix breaking changes introduced by the release of Poetry 2, as reported in #16 .
I suggest in order to support Poetry 2 in the future, to remove the version constraint from pip install "poetry<2" in /Dockerfile and make the necessary changes to pyproject.toml in a later commit and set a major release at that point to mark that deprecation

[tiagovrtr]

Any opinions on this @nborrmann ?