cmd/sketch: trim trailing / from skaband-addr

Otherwise the redirects that Go's mux does
makes for very confusing error messages.
diff --git a/cmd/sketch/main.go b/cmd/sketch/main.go
index 5c9436a..c4c978e 100644
--- a/cmd/sketch/main.go
+++ b/cmd/sketch/main.go
@@ -455,6 +455,8 @@
 		flags.mounts[i] = expanded + ":" + container
 	}
 
+	flags.skabandAddr = strings.TrimSuffix(flags.skabandAddr, "/")
+
 	return flags
 }