Update docker image build step in rl_on_mult_host documentation

[SurbhiJainUSC]

Description

• Update docker image build instructions for RL
• FIXES: b/490518973

Tests

N/A - documentation changes

Checklist

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• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[richjames0]

lgtm

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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