Enforce Poetry version

[rkorsak]

Feature Request

When poetry updates their tool, it can include breaking changes to the lock file parsing. We saw this today when poetry dropped an update and the automated build failed because it was using the latest poetry and unable to parse the lock file.

In the ideal solution:

• Running make environment for the first time would install a specific version of poetry (call it vX.Y)
• Running make environment on an existing environment would ensure the correct version of poetry is installed
• The docker container would use the correct poetry version in all cases
• Our linting or CI could include some verification that the lock file is generated for the correct version of poetry (I haven't looked deeply enough at the lock file to know how hard this would be).
• We can specify the poetry version in one place for easy maintenance

At a minimum, we need the first and third bullets.

[rkorsak]

We're going with the minimal solution for now - just manually specifying the poetry version in the Makefile and Dockerfile.