Install Poetry from pinned version

This longstanding TODO will at least make sure we have one source of
truth for Poetry versions.

Dockerfile

@@ -7,7 +7,7 @@
 # - Ensure that Celery works as well
 # - Run WSGI with an ENTRYPOINT
 
-FROM ubuntu:22.04 as build
+FROM ubuntu:22.04 AS build
 
 WORKDIR /app/
 
@@ -42,8 +42,8 @@ ENV LC_ALL=en_US.UTF-8
 
 RUN pip3 install --upgrade pip==22.0.4
 
-# TODO: Check asdf's .tool-versions
-RUN pip install poetry==1.7.1
+COPY .tool-versions .
+RUN pip install poetry==$(awk '/poetry/{printf $2}' .tool-versions)
 COPY poetry.lock .
 
 # Instruct poetry to create a venv in `.venv`, then auto-add it to path