sketch: set relpath at runtime, not build time

Previously, relpath was baked into the docker container at build time by setting
the working directory with 'docker create -w /app/relpath'. This was incorrect
because relpath can vary between different invocations of sketch.

Now:
- Remove -w flag from docker container creation
- Pass relpath as -C flag to sketch binary at runtime
- Container always starts with working directory /app (git root)
- Inner sketch process changes to correct directory at startup

This ensures sketch works correctly regardless of which subdirectory it's
invoked from, without hardcoding paths into the container image.

Co-Authored-By: sketch <hello@sketch.dev>
Change-ID: s2da1f8f867cd0e96k
diff --git a/dockerimg/dockerimg.go b/dockerimg/dockerimg.go
index b8fbd98..b800fd9 100644
--- a/dockerimg/dockerimg.go
+++ b/dockerimg/dockerimg.go
@@ -512,9 +512,6 @@
 	} else {
 		cmdArgs = append(cmdArgs, "-p", "0:22") // use an ephemeral host port for ssh.
 	}
-	if relPath != "." {
-		cmdArgs = append(cmdArgs, "-w", "/app/"+relPath)
-	}
 	// colima does this by default, but Linux docker seems to need this set explicitly
 	cmdArgs = append(cmdArgs, "--add-host", "host.docker.internal:host-gateway")
 
@@ -567,6 +564,9 @@
 	)
 	// Set SSH connection string based on session ID for SSH Theater
 	cmdArgs = append(cmdArgs, "-ssh-connection-string=sketch-"+config.SessionID)
+	if relPath != "." {
+		cmdArgs = append(cmdArgs, "-C", relPath)
+	}
 	if config.Model != "" {
 		cmdArgs = append(cmdArgs, "-model="+config.Model)
 	}