docs: update Docker spawn example

[zerone0x]

Summary

Fixes #343

Update the README Docker auto-spawn example so it works with the current opencode container image.

Changes

• Use https://localhost for the local Docker server URL.
• Use the official ghcr.io/anomalyco/opencode:latest image.
• Pass serve --port 4096 --hostname '0.0.0.0' directly to the image instead of repeating the opencode command.

Test Plan

• Verified README Markdown code fences are balanced.
• Checked the updated example contains the requested image/command and no longer contains the old opencode:latest opencode serve command.

[sudo-tee]

Thanks for the docs update