Fix Dockerfile for working with local previews

[crd477]

Description

The Dockerfile that we provide for working with local previews of the website content was based an old Jekyll container image. This container image fails to build the site with the following error after a recent required update to the github-pages dependency:

Bundler could not find compatible versions for gem "ruby":
  In Gemfile:
    ruby

    faraday (= 2.10.1) was resolved to 2.10.1, which depends on
      ruby (>= 3.0)

    github-pages (= 232) was resolved to 232, which depends on
      ruby (>= 2.3.0)

This PR updates the Dockerfile to use a newer Jekyll container image and updates the relevant documentation about using containers for local website development.

The jekyll/jekyll:stable image that this change is based on is also a bit stale at this point (it was last updated 21 months ago), but at least it allows us to achieve parity with the versions of Jekyll and github-pages used by GitHub Pages for the live site.

Checklist:

• I have previewed changes locally or with CircleCI (runs when PR is created)
• I have completed any content reviews, such as getting input from relevant working groups. If no, please note this and wait to post the PR to the #website channel until the content has been settled.

When you are ready for a technical review/merge, post the for the link for the PR in the US-RSE Slack (#website) to ask for reviewers.

[mrmundt]

Major kudos for spending time to figure this out!