.go

	// Optional: no more than one of the following may be specified.
	// Optional: Defaults to ""; variable references $(VAR_NAME) are expanded
	// using the previous defined environment variables in the container and
	// any service environment variables.  If a variable cannot be resolved,
	// the reference in the input string will be unchanged.  The $(VAR_NAME)
	// syntax can be escaped with a double $$, ie: $$(VAR_NAME).  Escaped 
	// references will never be expanded, regardless of whether the variable
	// exists or not.