fix(build): correct docker build process for mobile sandbox

[XiuShenAl]

Description

Correct docker build process for mobile sandbox

[Copilot]

Pull request overview

This PR refactors the Docker build process for the mobile sandbox to avoid Docker-in-Docker (DinD) during the image build phase. Instead of pulling the Redroid image inside the Dockerfile, the build script now prepares the Redroid image externally and copies it into the build context as a tarball.

Key changes:

• Extracts Redroid image preparation from Dockerfile to Python build script
• Consolidates host environment checking into a dedicated module
• Implements a two-phase container startup (health check vs normal mode) to preserve the Redroid tarball during image commit

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 10 comments.

Show a summary per file

File	Description
src/agentscope_runtime/sandbox/build.py	Adds prepare_redroid_image function to pull and save Redroid image externally; integrates host readiness checks for mobile builds; modifies build_image to handle mobile-specific requirements
src/agentscope_runtime/sandbox/box/mobile/mobile_sandbox.py	Removes duplicate host checking code and delegates to new host_checker module
src/agentscope_runtime/sandbox/box/mobile/box/host_checker.py	New module consolidating host environment validation logic previously embedded in mobile_sandbox.py
src/agentscope_runtime/sandbox/box/mobile/box/scripts/start.sh	Adds run-mode detection to conditionally preserve redroid.tar during health checks; verifies Redroid image is loaded
src/agentscope_runtime/sandbox/box/mobile/Dockerfile	Removes redroid-fetcher stage that used DinD; copies redroid.tar from build context instead
.github/workflows/build_sandbox_image.yml	Adds step to load Android kernel modules before build

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