## Objective

Fixes #63227 — `.dockerignore` files display a generic icon instead of
the Docker icon.

## Solution

Added `.dockerignore` to the list of filenames mapped to the Docker icon
in `crates/theme/src/icon_theme.rs`.

## Testing

- No automated tests added; the change is a static mapping.
- Manually verified by opening a project with a `.dockerignore` file in
Zed.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments (none)
- [x] The content adheres to Zed's UI standards (single icon mapping)
- [ ] Tests cover the new/changed behavior (not required for static
mapping)
- [x] Performance impact has been considered and is acceptable

## Showcase

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- `.dockerignore` now shows the Docker icon for the bundled icon theme.