[19.03] vendor: moby/buildkit v0.6.4-20-g4cb720ef

full diff: moby/buildkit@dc6afa0...4cb720e

- contenthash: ignore system and security xattrs in calculation
    - fixes moby/buildkit#1330 COPY cache not re-used depending on SELinux environment
    - fixes moby/moby#39003 (comment)
- contenthash: allow security.capability in cache checksum
- inline cache: fix handling of duplicate blobs
    - fixes moby/buildkit#1388 cache-from working unreliably

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
Upstream-commit: 23d47bd12eaeeb93bbc4e9e80020c811e9eb2980
Component: engine

Author: thaJeztah

components/engine/vendor.conf

@@ -26,7 +26,7 @@ github.com/imdario/mergo                            7c29201646fa3de8506f70121347
 golang.org/x/sync                                   e225da77a7e68af35c70ccbf71af2b83e6acac3c
 
 # buildkit
-github.com/moby/buildkit                            dc6afa0f755f6cbb7e85f0df4ff4b87ec280cb32 # v0.6.4-15-gdc6afa0f
+github.com/moby/buildkit                            4cb720ef6483b43020c9037726de47353ac8ad9b # v0.6.4-20-g4cb720ef
 github.com/tonistiigi/fsutil                        6c909ab392c173a4264ae1bfcbc0450b9aac0c7d
 github.com/grpc-ecosystem/grpc-opentracing          8e809c8a86450a29b90dcc9efbf062d0fe6d9746
 github.com/opentracing/opentracing-go               1361b9cd60be79c4c3a7fa9841b3c132e40066a7