chore(deps): cleanup go.mod a little bit (#136)

* Upgrade docker to v19.03.7

* go mod vendor

* golangci-lint 1.23.8

vendor/github.com/containerd/continuity/sysx/README.md

@@ -0,0 +1,3 @@
+This package is for internal use only. It is intended to only have
+temporary changes before they are upstreamed to golang.org/x/sys/
+(a.k.a. https://github.com/golang/sys).