Vendor in latest containers/buildah

Switch from projectatomic/buildah to containers/buildah

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

vendor/github.com/containers/buildah/run_linux.go

@@ -0,0 +1,17 @@
+// +build linux
+
+package buildah
+
+import (
+	"fmt"
+	"golang.org/x/sys/unix"
+	"os"
+)
+
+func setChildProcess() error {
+	if err := unix.Prctl(unix.PR_SET_CHILD_SUBREAPER, uintptr(1), 0, 0, 0); err != nil {
+		fmt.Fprintf(os.Stderr, "prctl(PR_SET_CHILD_SUBREAPER, 1): %v\n", err)
+		return err
+	}
+	return nil
+}