The compose command creates a new object whose content is the concatenation
of a given sequence of source objects under the same bucket. gsutil uses
the content type of the first source object to determine the destination
object's content type and does not modify or delete the source objects as
part of the compose command. For more information, see the composite objects
topic.
There is a limit (currently 32) to the number of components that can
be composed in a single operation.