sandbox.py: Include Dockerfile path in missing dependency error messages

[coderabbitai]

Description

When sandbox.py reports missing files referenced in a Dockerfile, the error message does not indicate which Dockerfile is being processed. This context could be helpful when working with multiple Dockerfiles, build args, or multi-stage builds.

Enhancement Request

Enhance the error message in setup_sandbox() to include the Dockerfile path that was being processed when the missing dependency was detected.

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