chore: Add note for container_user

[mqasimsarfraz]

When running the server via MCP toolkit on docker-engine, users need to configure container_user to ensure the gateway works fine. Since the upstream PRs have been merged we can go ahead and include this in our doucmentation:

• feat: Add support for User docker/mcp-registry#180
• Feat: Support for User docker/mcp-gateway#104

[mqasimsarfraz]

cc: @pauldotyu

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
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Additional details and impacted files

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  Files           ?       61           
  Lines           ?     6277           
  Branches        ?        0           
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  Hits            ?     2228           
  Misses          ?     3954           
  Partials        ?       95           

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[feiskyer]

LGTM, thanks for the notes here