Update README to match current dev/build workflow

[bernalde]

Summary

• remove outdated steps to manually create Dockerfile and Gemfile
• align Docker run/build commands with tracked repo files and image name
• add optional native Ruby workflow
• add a clear validate.sh pre-PR step
• remove contradictory note about Dockerfile/Gemfile being gitignored

Why

Keeps README consistent with the actual repository setup and reduces onboarding friction.

Closes #157

[Copilot]

Pull request overview

Updates the repository’s README so local development and build instructions match the current, tracked Docker/Ruby setup and reduce onboarding friction (per #157).

Changes:

• Removes outdated instructions to manually create Dockerfile/Gemfile and the contradictory note about them being gitignored.
• Aligns Docker image name/commands to secquoia-website, including improved run flags.
• Adds optional native Ruby workflow and a pre-PR validate.sh step.

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[bernalde]

@ahribljan I assigned you as a reviewer of this PR. Verify that the new instructions in README are valid and document any comments you have in a review here