dockerimg: +SSHTheater, auto-configures local ssh

- removes the old cli flags for pointing sketch at existing ssh key files
- generates key pairs for both the container ssh server and the user if
  these don't already exist in ~/.sketch/
- automatically edits ~/.sketch/known_hosts and ~/.sketch/ssh_config to
  add entries when you start a sketch container and remove them when you're
  done
- does not make any edits to default ~/.ssh/config files, so you'll have
  to manually add line to you default ssh config file like:
  `Include HOME_DIR/.sketch/ssh_config`
  in order to have your ssh tools pick up what sketch is putting down.
diff --git a/go.mod b/go.mod
index 6d2bb2d..329b50d 100644
--- a/go.mod
+++ b/go.mod
@@ -20,6 +20,7 @@
 require (
 	github.com/anmitsu/go-shlex v0.0.0-20200514113438-38f4b401e2be // indirect
 	github.com/gliderlabs/ssh v0.3.8 // indirect
+	github.com/kevinburke/ssh_config v1.2.0 // indirect
 	github.com/mattn/go-colorable v0.1.13 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	golang.org/x/mod v0.24.0 // indirect