Cannot Marshal a `command` as a single string.

[mattdee123]

The command field of a compose service can be either a list of strings, or a single string.

This project represents it as

// ShellCommand is a string or list of string args
type ShellCommand []string

However, there doesn't seem to be a way to marshal it as a single string --it always comes out as a list. Does this mean that only the list form is supported?