readonly mount

the `mount` property in the `devcontainer.json` as described here https://containers.dev/implementors/json_reference/#general-properties accepts a list of `:`. How can I flag the mount to be `readonly` using this type of definition?

Most often, the mount instruction is a one-liner (https://code.visualstudio.com/remote/advancedcontainers/add-local-file-mount), and in that case I can specify the `readonly` flag like

```
"mounts": ["source=...,target=...,type=bind,readonly]
```