1. A computer-implemented method, comprising: under the control of one or more computer systems configured with executable instructions, creating a cluster and a cluster identifier that identifies the cluster, wherein the cluster represents a set of container instances associated with a customer of a computing resource service provider that are registered to the cluster and configured to run one or more software containers; obtaining an image of a software container configured to execute within a computer system instance, wherein the software container comprises data and one or more processes which, when executed in the computer system instance, are isolated from other processes running within the computer system instance, and wherein the image is an entire copy of a particular state of the software container at a time the image was generated; receiving a request to launch a set of images, including the image of the software container, in accordance with a task definition, wherein the task definition identifies a set of software containers, including the software container, that are assigned to start as a group and specifies an allocation of resources to the set of software containers, wherein the request specifies the cluster identifier of the cluster; and in response to the request to launch the one or more images, determining a subset of the set of container instances represented by the cluster specified by the cluster identifier in which to launch the set of software containers, wherein one or more container instances are determined according to a placement scheme; and launching the set of images to yield running software containers within the subset of the set of container instances so that the running software containers are allocated resources in accordance with the allocation of resources specified in the task definition.