Fix poetry install issue on platform.sh.

`poetry install` raised: `AttributeError: 'Link' object has no attribute
'is_absolute'` during build on platform.sh.

It appears that it's possible for poetry-core to update even if poetry
is locked. Our issue wasa caused by a regression introduced in 1.0.4.

Updated poetry following the advice in the linked github issue and
updated pip to fix an issue selecting the correct wheel for cryptography
36.0.0 on platform.sh.

Locked Poetry to 1.1.12 and pip to 21.3.1 for docker and platform.sh

Related issues:

 - [Link object being passed instead of Path to poetry.core.packages.file_dependency.FileDependency in poetry-core 1.0.5](python-poetry/poetry#4541)
 - [fix: Broken cache on Windows #4531](python-poetry/poetry#4531)

.platform.app.yaml

@@ -15,7 +15,8 @@ variables:
 
 dependencies:
   python3:
-    poetry: '1.1.4'
+    poetry: '1.1.12'
+    pip: '21.3.1'
 
 # The size of the persistent disk of the application (in MB).
 disk: 2048

Dockerfile

@@ -8,7 +8,8 @@ WORKDIR /app/
 
 COPY poetry.lock /app/
 COPY pyproject.toml /app/
-RUN pip install poetry
+RUN python -m pip install pip==21.3.1
+RUN pip install poetry==1.1.12
 RUN poetry install
 
 EXPOSE 8000