Fix deprecated Poetry installer

[nickrecchi]

Dockerfile for api-v3 was changed to use https://install.python-poetry.org after encountering an error due to get-poetry.py being deprecated. After applying this change, API v3 runs successfully and API functionality was confirmed to be working properly.

Here's the error that occurs after running docker-compose up:

failed to solve: executor failed running [/bin/sh -c curl -sSL https://raw.githubusercontent.com/python-poetry/poetry/master/get-poetry.py | POETRY_HOME=/opt/poetry python &&     cd /usr/local/bin &&     ln -s /opt/poetry/bin/poetry &&     poetry config virtualenvs.create false]: exit code: 1

Further details on the error indicate the installer that should be used:

This installer is deprecated, and scheduled for removal from the Poetry repository on or after January 1, 2023.
See https://github.com/python-poetry/poetry/issues/6377 for details.

You should migrate to https://install.python-poetry.org instead, which supports all versions of Poetry, and allows for `self update` of versions 1.1.7 or newer.
Instructions are available at https://python-poetry.org/docs/#installation.

Here's the issue describing the removal of get-poetry.py:
python-poetry/poetry#6377

[johnseekins]

Thanks for this PR, @nickrecchi . I think we're going to try and use the fix in #26 though, as it's a bit more future-proof.