docker: use pip install + poetry export instead of poetry update

I dunno man, poetry just seems like a mess.

contrib/docker/scripts/build.sh

@@ -23,8 +23,8 @@ export LIGHTNINGD_POSTGRES_NO_VACUUM=1
 
 pip3 install --upgrade pip
 pip3 install --user poetry
-poetry config virtualenvs.create false --local
-poetry install
+poetry export --dev --without-hashes -o requirements.txt
+pip3 install -r requirements.txt
 
 git clone https://github.com/lightningnetwork/lightning-rfc.git ../lightning-rfc
 git submodule update --init --recursive