docker: upgrade python release to fix dtschema build error

docker/Dockerfile

 # use debian base
-FROM debian:buster-slim
+FROM python:slim-buster
 
 # apt proxy (optional)
 ARG APTPROXY=
@@ -14,7 +14,7 @@ RUN set -e; \
 	apt-get -y upgrade; \
 	:
 
-RUN apt-get update ; apt-get -y install --no-install-recommends acpica-tools bc bison build-essential ca-certificates cpio debian-archive-keyring debootstrap device-tree-compiler dosfstools e2tools fakeroot file flex fuseext2 g++ gcc git kmod libncurses-dev libssl-dev libyaml-dev make meson mtools multistrap ninja-build p7zip p7zip-full pandoc parted python python2 python3 qemu-system-arm rsync squashfs-tools sudo tar u-boot-tools unzip wget xz-utils yamllint
+RUN apt-get update ; apt-get -y install --no-install-recommends acpica-tools bc bison build-essential ca-certificates cpio debian-archive-keyring debootstrap device-tree-compiler dosfstools e2tools fakeroot file flex fuseext2 g++ gcc git kmod libncurses-dev libssl-dev libyaml-dev make meson mtools multistrap ninja-build p7zip p7zip-full pandoc parted pkg-config python python2 python3 qemu-system-arm rsync squashfs-tools sudo tar u-boot-tools unzip wget xz-utils yamllint
 
 RUN set -e; \
 	apt-get -y install --no-install-recommends libpython3-dev python3-pip python3-setuptools python3-wheel swig; \