golangci-lint: depguard: prevent uses of pkg/system

We no longer use this package; add a linter rule to prevent
accidentally importing it again.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

.golangci.yml

@@ -59,6 +59,8 @@ linters-settings:
             desc: Use github.com/moby/sys/userns instead.
           - pkg: "github.com/containerd/containerd/platforms"
             desc: The containerd platforms package was migrated to a separate module. Use github.com/containerd/platforms instead.
+          - pkg: "github.com/docker/docker/pkg/system"
+            desc: This package should not be used unless strictly necessary.
           - pkg: "io/ioutil"
             desc: The io/ioutil package has been deprecated, see https://go.dev/doc/go1.16#ioutil
   gocyclo: