Configured Poetry to not use virtualenvs in ./.venv

Having a virtualenv in `.venv` is very convenient because many tools
automatically pick up on it. However, this then also happens during the
construction of the Docker images, which subsequently breaks.

Until a proper fix is found, it's easiest to just put the virtualenv
outside of the project.

poetry.toml

@@ -0,0 +1,2 @@
+[virtualenvs]
+in-project = false