Include feature-flags.dev.json in backend Docker build context

[aapeliv]

The backend .dockerignore ignores everything and only allow-lists specific paths. feature-flags.dev.json (added in 892f073, "Switch to growthbook feature flags") was not in the allow-list, so COPY . /app in the Dockerfile silently excluded it from the build context. Because backend.dev.env sets FEATURE_FLAGS_FILE_OVERRIDE_PATH=feature-flags.dev.json, the backend crashed on startup with FileNotFoundError: .../feature-flags.dev.json — and rebuilding with docker compose up --build didn't help, since the file was never in the build context to begin with.

This adds !feature-flags.dev.json to the allow-list so it gets copied into the image.

Testing

• docker compose up --build and confirm the backend starts without the FileNotFoundError from setup_experimentation().

Backend checklist

• Added tests for any new code or added a regression test if fixing a bug
• Run the backend locally and it works
• Added migrations if there are any database changes, rebased onto develop if necessary for linear migration history

For maintainers

• Maintainers can push commits to my branch
• Maintainers can merge this PR for me

This PR was created with the Couchers PR skill.

[CouchersBot]

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Deep links

iOS

couchers-devtool://expo-development-client/?url=https%3A%2F%2Fc089d97a--ota.preview.couchershq.org%2Fios%2Fmanifest

Android

couchers-devtool://expo-development-client/?url=https%3A%2F%2Fc089d97a--ota.preview.couchershq.org%2Fandroid%2Fmanifest

Web (Vercel)

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Backend

Schema	Schema diff	Sample emails

Other

Protos	Backend coverage	Web coverage

[nabramow]

LGTM!

[github-actions]

📝 Release Notes

This PR does not need to be included in release notes.

Reason: This fixes a developer/docker build configuration issue that caused the backend to fail only in a local containerized development setup. It is not a user-facing change, and it is not major technical infrastructure work that end users would notice in production.

🤖 Bot Debug Information

Model: couchers.openai.gpt-5.4
Decision: exclude
Reasoning: This fixes a developer/docker build configuration issue that caused the backend to fail only in a local containerized development setup. It is not a user-facing change, and it is not major technical infrastructure work that end users would notice in production.