Merge pull request #498 from thaJeztah/19.03_backport_bump_buildx

[19.03 backport] bump buildx to v0.4.2
Upstream-commit: 395008c8af7a26ba5980f074bfdcb205b1651950
Component: packaging

Author: silvin-lubecki

components/packaging/plugins/buildx.installer

@@ -6,7 +6,7 @@ source "$(dirname "$0")/.common"
 PKG=github.com/docker/buildx
 GOPATH=$(go env GOPATH)
 REPO=https://${PKG}.git
-: "${BUILDX_COMMIT=v0.3.1}"
+: "${BUILDX_COMMIT=v0.4.2}"
 DEST=${GOPATH}/src/${PKG}
 
 build() {