Fix #1143: Add content hash to build context metadata to prevent BuildKit from caching stale files

[Ronitsabhaya75]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Fix #1143

BuildKit container caches build context after the first build and doesn't invalidate when files change, causing subsequent builds to use stale files.

Root Cause: BuildKit's cache key doesn't account for file content changes. BuildFSSync sends updated tar archives, but BuildKit reuses the cached context.

The fix: Add SHA256 hash of build context (file paths + sizes + modTimes) to tar metadata, forcing BuildKit cache invalidation when files change.

let contextHash = try computeContextHash(fileOrder: fileOrder, contextDir: contextDir)
metadata: [
    "os": "linux",
    "stage": "fssync",
    "mode": "tar",
    "context_hash": contextHash,
]

Testing

• Tested locally
• Added/updated tests
• Added/updated docs