add python3, update dependencies, update documentation

[davad]

My team needs to have Python for part of our CI/CD pipeline that uses this image.

I saw someone else requested adding it back (https://github.com/zenika-open-source/terraform-aws-cli/issues/80), so I put together a PR for it.

While testing I saw that a couple system packages didn't have the old pinned version available in their package repository anymore.

[davad]

I noticed this doesn't have pip. It might also be missing some common packages that https://github.com/zenika-open-source/terraform-aws-cli/issues/80 needs for the Lambda integration.

Let me know if this is something the maintainers are interested in, and I can update the PR further.

[davad]

If anyone else needs this, we ended up building our own image like this:

ARG AWS_CLI_VERSION=2.14.5
ARG RELEASE_VERSION=8.0
ARG TERRAFORM_VERSION=1.6.5
FROM zenika/terraform-aws-cli:release-${RELEASE_VERSION}_terraform-${TERRAFORM_VERSION}_awscli-${AWS_CLI_VERSION}

USER root

ARG PYTHON_MAJOR_VERSION=3.11
RUN apt-get update \
  && apt-get install -y --no-install-recommends \
     python3=${PYTHON_MAJOR_VERSION}.2-1+b1 \
     python3-pip=23.0.1+dfsg-1 \
     python3-urllib3=1.26.12-1 \
  && apt-get clean \
  && rm -rf /var/lib/apt/lists/* \
  && update-alternatives --install /usr/bin/python python /usr/bin/python${PYTHON_MAJOR_VERSION} 1


WORKDIR /workspace
USER nonroot