Adding pattern for enforcing readonly root filesystem

[nathanpeck]

Description

Checklist

Please make sure you've completed the relevant tasks for this PR, out of the following list:

• [ ] Understand the repository structure
• [ ] Add yourself as an author
• [ ] Add your pattern
• [ ] Add a great SVG diagram for your pattern
• [ ] Read our general contribution guidelines
• [ ] Read our code of conduct
• [ ] Confirm your pattern frontmatter lints correctly. i.e. run npm install && npm run lint locally from your dev environment

[containers-aws-bot]

container-patterns PR Release triggered by aws-samples

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Commit ref: readonly-root-filesystem Commit sha: ba425d39db40e705671e1f3fb897c9201f8b47d4

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[containers-aws-bot]

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