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.sum b/go.sum
index 2741a4a..7892add 100644
--- a/go.sum
+++ b/go.sum
@@ -14,6 +14,8 @@
 github.com/go-quicktest/qt v1.101.0/go.mod h1:14Bz/f7NwaXPtdYEgzsx46kqSxVwTbzVZsDC26tQJow=
 github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
 github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
+github.com/kevinburke/ssh_config v1.2.0 h1:x584FjTGwHzMwvHx18PXxbBVzfnxogHaAReU4gf13a4=
+github.com/kevinburke/ssh_config v1.2.0/go.mod h1:CT57kijsi8u/K/BOFA39wgDQJ9CxiF4nAY/ojJ6r6mM=
 github.com/kr/pretty v0.3.1 h1:flRD4NNwYAUpkphVc1HcthR4KEIFJ65n8Mw5qdRn3LE=
 github.com/kr/pretty v0.3.1/go.mod h1:hoEshYVHaxMs3cyo3Yncou5ZscifuDolrwPKZanG3xk=
 github.com/kr/text v0.2.0 h1:5Nx0Ya0ZqY2ygV366QzturHI13Jq95ApcVaJBhpS+AY=