[R] Unify usernames with vscode.

Since the old username `docker` is the default username in old image
r-apt:bionic, and the new image r-ver have no default user.
https://github.com/rocker-org/rocker/blob/master/r-apt/bionic/Dockerfile

containers/r/.devcontainer/devcontainer.json

@@ -36,5 +36,5 @@
 	// "runArgs": [ "--cap-add=SYS_PTRACE", "--security-opt", "seccomp=unconfined" ],
 
 	// Comment out connect as root instead. More info: https://aka.ms/vscode-remote/containers/non-root.
-	"remoteUser": "docker"
+	"remoteUser": "vscode"
 }