Claude Code devcontainer feature silently overwrites custom /usr/local/bin/init-firewall.sh

## What happened

The ghcr.io/anthropics/devcontainer-features/claude-code:1.0 feature installs its own init-firewall.sh to /usr/local/bin/init-firewall.sh. Because devcontainer features run after the Dockerfile build, this silently overwrites any user-provided file at the same path. There is no warning, log message, or documentation about this behavior.

## Steps to reproduce

1. Create a `.devcontainer/Dockerfile` that copies a custom firewall script:
```dockerfile
COPY init-firewall.sh /usr/local/bin/init-firewall.sh
RUN chmod +x /usr/local/bin/init-firewall.sh
```
2. Include the feature in `devcontainer.json`:
```json
"features": {
"ghcr.io/anthropics/devcontainer-features/claude-code:1.0": {}
}
```
3. Build the devcontainer.
4. Run `cat /usr/local/bin/init-firewall.sh` inside the container — the content is the feature's script, not the user's.

## Why this matters

The Claude Code [[devcontainer documentation](https://docs.anthropic.com/en/docs/claude-code/devcontainer)](https://docs.anthropic.com/en/docs/claude-code/devcontainer) describes `init-firewall.sh` as a component users should customize for their needs. Users following this guidance will copy and modify the reference `init-firewall.sh` in their own Dockerfile, only to have it silently replaced by the feature's version every build. Using a different filename works around the issue, but users shouldn't need to discover this through trial and error when the docs encourage modifying this exact file.