Add a new `astro dev build` CLI command that only builds the Docker image
without starting the local Airflow environment. This is useful for:
- Testing that a project builds successfully before deployment
- Preparing an image for deployment workflows
- Faster iteration when only the build step is needed

The command supports the following flags:
- --no-cache: Build without using Docker cache
- --image-name: Tag a custom image as the project image
- --build-secrets: Pass build secrets to the build process

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>