Update Dockerfile to use Python 3.12-bookworm variant

Author: aditya-chouksey

.devcontainer/Dockerfile

@@ -1,5 +1,5 @@
 # https://github.com/microsoft/vscode-dev-containers/blob/main/containers/python-3/README.md
-ARG VARIANT=3.13-bookworm
+ARG VARIANT=3.12-bookworm
 FROM mcr.microsoft.com/vscode/devcontainers/python:${VARIANT}
 COPY requirements.txt /tmp/pip-tmp/
 RUN python3 -m pip install --upgrade pip \