Ignore SIGURG on Linux.

In go1.14+, SIGURG is used by the runtime to handle preemtable system
calls.
In practice this signal caught *frequently*.

For reference:

https://go.googlesource.com/proposal/+/master/design/24543-non-cooperative-preemption.md
https://github.com/golang/go/issues/37942

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

cli/command/container/client_test.go

@@ -32,6 +32,7 @@ type fakeClient struct {
 	containerExportFunc     func(string) (io.ReadCloser, error)
 	containerExecResizeFunc func(id string, options types.ResizeOptions) error
 	containerRemoveFunc     func(ctx context.Context, container string, options types.ContainerRemoveOptions) error
+	containerKillFunc       func(ctx context.Context, container, signal string) error
 	Version                 string
 }
 
@@ -154,3 +155,10 @@ func (f *fakeClient) ContainerExecResize(_ context.Context, id string, options t
 	}
 	return nil
 }
+
+func (f *fakeClient) ContainerKill(ctx context.Context, container, signal string) error {
+	if f.containerKillFunc != nil {
+		return f.containerKillFunc(ctx, container, signal)
+	}
+	return nil
+}