Update README.md

[dkwon17]

What does this PR do?

Mention the optional export DOCKER=podman flag.

What issues does this PR fix or reference?

Is it tested? How?

PR Checklist

• E2E tests pass (when PR is ready, comment /test v8-devworkspace-operator-e2e, v8-che-happy-path to trigger)
  • v8-devworkspace-operator-e2e: DevWorkspace e2e test
  • v8-che-happy-path: Happy path for verification integration with Che

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 39.57%. Comparing base (46ecfee) to head (ff6ae8c).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1446      +/-   ##
==========================================
- Coverage   39.58%   39.57%   -0.01%     
==========================================
  Files         160      160              
  Lines       13178    13186       +8     
==========================================
+ Hits         5216     5219       +3     
- Misses       7586     7590       +4     
- Partials      376      377       +1     

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[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dkwon17, rohanKanojia

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[dkwon17]

Closing in favour of #1450