Update README to incorporate using existing docker image

[yuandong-tian]

Why are these changes needed?

To enable user to use AutoGen directly via a downloadable docker image.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a122ffe) 26.53% compared to head (f249d57) 26.53%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1040   +/-   ##
=======================================
  Coverage   26.53%   26.53%           
=======================================
  Files          28       28           
  Lines        3807     3807           
  Branches      865      865           
=======================================
  Hits         1010     1010           
  Misses       2726     2726           
  Partials       71       71           

Flag	Coverage Δ
unittests	26.47% <ø> (ø)

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[qingyun-wu]

Thanks @yuandong-tian. The addition looks good. Could you run pre-commit to fix format issues? Find instructions here: https://microsoft.github.io/autogen/docs/Contribute#pre-commit

[qingyun-wu]

Thanks @yuandong-tian. The addition looks good. There are some minor format issues: https://github.com/microsoft/autogen/actions/runs/7293310300/job/19876097044?pr=1040
Could you run pre-commit to fix format issues? Find instructions here: https://microsoft.github.io/autogen/docs/Contribute#pre-commit