Make bind address configurable for app dev server

[lmcrean]

Adds configurable bind address support to the shopify app dev command to enable Docker container development workflows.

Problem

• Fixes [Feature]: Make bind address configurable for local servers #6355
• Recent CLI versions hardcode localhost binding for app dev servers for security
• Docker containers require 0.0.0.0 binding to accept connections from host machine
• Theme dev server already supports --host flag, but app dev server does not
• Breaks containerized development setups (working version was 3.83.3)

Solution

• Add --host flag to shopify app dev command with SHOPIFY_FLAG_HOST env var support
• Update proxy server setup to use configurable host instead of hardcoded 'localhost'
• Maintain secure default (127.0.0.1) while enabling Docker flexibility
• Match existing theme dev server pattern for consistency

Changes Made

1. Added host flag to app dev command

• File: packages/app/src/cli/commands/app/dev.ts
• Added --host flag with environment variable support
• Default: 127.0.0.1 (maintains security)
• Override: 0.0.0.0 (enables Docker containers)

2. Updated proxy server to use configurable host

• File: packages/app/src/cli/services/dev/processes/setup-dev-processes.ts
• Changed hardcoded 'localhost' to use passed host parameter
• Added host parameter to proxy server setup function

Usage Examples

For Docker containers:

SHOPIFY_FLAG_HOST=0.0.0.0 shopify app dev
# or
shopify app dev --host=0.0.0.0

For normal development (default):

shopify app dev  # Still binds to localhost by default

Testing

Unit Tests

Added comprehensive unit test coverage in packages/app/src/cli/services/dev/processes/setup-dev-processes.test.ts:

New Test: proxy server process includes host parameter when configured for Docker

• Purpose: Verifies that the proxy server correctly uses the host parameter when set to 0.0.0.0 for Docker compatibility
• Test setup: Creates a dev configuration with host: '0.0.0.0' simulating Docker usage
• Verification: Confirms the proxy server process options include the correct host value
• Location: Lines 85-150 in the test file

Updated Existing Tests

All existing tests were updated to include the required host: 'localhost' parameter in commandOptions to satisfy TypeScripts' requirements for the Devoptions interface. Users get localhost by default so no action needed.

Test Commands

# Run unit tests for the setup-dev-processes module
pnpm test packages/app/src/cli/services/dev/processes/setup-dev-processes.test.ts

Manual Testing

• Verify default behavior unchanged (binds to localhost)
• Verify --host=0.0.0.0 enables external connections
• Verify SHOPIFY_FLAG_HOST environment variable works
• Test Docker container setup with port forwarding
• Ensure backwards compatibility

No Breaking Changes

This is a feature addition that maintains 100% backward compatibility. Users who don't need Docker support will see zero changes in behavior. Only users who specifically want to bind to a different network interface need to use the new --host flag.

Previously #6396; this PR appears ready to merge