Sketch "inception" support.

I wanted to tell Sketch-in-container to run

	go run ./cmd/sketch -unsafe -skaband-addr= -one-shot 'what is todays date'"

and have it work. And now it does.

The underlying complexity is whether "inDocker" is enough to know
"isInsideSketch", so I separated that as far as necessary. The skaband
stuff is still complicated.

It might be preferable to use env variables for all outside->inside
communication rather than use flags sometimes and variables sometimes
(like, for keys), but here we are.
diff --git a/cmd/sketch/main.go b/cmd/sketch/main.go
index 8ea4b65..ae029f9 100644
--- a/cmd/sketch/main.go
+++ b/cmd/sketch/main.go
@@ -117,6 +117,7 @@
 	if _, err := os.Stat("/.dockerenv"); err == nil {
 		inDocker = true
 	}
+	inInsideSketch := inDocker && *outsideHostname != ""
 
 	if !inDocker {
 		msgs, err := hostReqsCheck(*unsafe)
@@ -256,7 +257,7 @@
 		return err
 	}
 
-	if !inDocker {
+	if !inInsideSketch {
 		ini := loop.AgentInit{
 			WorkingDir: wd,
 		}
@@ -283,7 +284,7 @@
 		ps1URL = fmt.Sprintf("http://%s", ln.Addr())
 	}
 
-	if inDocker {
+	if inInsideSketch {
 		<-agent.Ready()
 		if ps1URL == "" {
 			ps1URL = agent.URL()