sandbox.py: Collect and report all missing Dockerfile dependencies at once

[coderabbitai]

Description

Currently, sandbox.py exits on the first missing file referenced in a Dockerfile. This means developers must fix issues one at a time, re-running the tool after each fix.

Enhancement Request

Modify the validation logic in setup_sandbox() to collect all missing files before exiting, then display them all in a single error message. This would allow developers to see the complete list of issues and fix them all at once.

Context

This enhancement was identified during the review of PR #3017, which implemented the initial fix for issue #967.

References

• Original fix PR: ISSUE #967: fix(sandbox): improve error reporting for missing Dockerfile dependencies #3017
• Discussion: ISSUE #967: fix(sandbox): improve error reporting for missing Dockerfile dependencies #3017 (comment)
• Original issue: need better reporting for files mentioned in Dockerfile but missing on disk #967

Requested by: @jiridanek