sketch: initial container ssh support (#15)

Adds an in-process ssh server to the sketch agent running inside
the container.

The ssh server implementation uses https://github.com/gliderlabs/ssh/

This change does not automatically generate any keys (this may come later).
You specify the server identity private key and the user's authorized public
keys on the sketch command line.

The host sketch process reads these files from the cli flags at startup. Once
the container is launched, it passes these keys to to the container
sketch process via new /init POST body fields.
diff --git a/go.mod b/go.mod
index 738d758..6d2bb2d 100644
--- a/go.mod
+++ b/go.mod
@@ -8,18 +8,21 @@
 	github.com/fatih/color v1.18.0
 	github.com/google/go-cmp v0.7.0
 	github.com/richardlehane/crock32 v1.0.1
+	go.skia.org/infra v0.0.0-20250421160028-59e18403fd4a
+	golang.org/x/crypto v0.37.0
 	golang.org/x/net v0.38.0
-	golang.org/x/sync v0.12.0
-	golang.org/x/term v0.30.0
+	golang.org/x/sync v0.13.0
+	golang.org/x/term v0.31.0
 	golang.org/x/tools v0.31.0
 	mvdan.cc/sh/v3 v3.11.0
 )
 
 require (
+	github.com/anmitsu/go-shlex v0.0.0-20200514113438-38f4b401e2be // indirect
+	github.com/gliderlabs/ssh v0.3.8 // indirect
 	github.com/mattn/go-colorable v0.1.13 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
-	go.skia.org/infra v0.0.0-20250421160028-59e18403fd4a // indirect
 	golang.org/x/mod v0.24.0 // indirect
-	golang.org/x/sys v0.31.0 // indirect
-	golang.org/x/text v0.23.0 // indirect
+	golang.org/x/sys v0.32.0 // indirect
+	golang.org/x/text v0.24.0 // indirect
 )