"When the source being documented forms a part of a larger whole, the larger whole can be thought of as a container that holds the source. The container is crucial to the identification of the source. The title of the container is normally italicized and is followed by a comma, since the information that comes next describes the container." (MLA Handbook, 8th ed., p. 30)
A container can also be nested within a larger container (e.g. a journal article is contained within a journal and that journal is contained within an online database.
Williams, George R. "What Can Consciousness Anomalies Tell Us about
Quantum Mechanics?" Journal of Scientific Exploration, vol.30,
no. 3, 2015, pp. 326-354. Academic Search Complete, 0-search.
6 Nov. 2016.
MLA Handbook, 8th ed. contains more information on containers on pages 30-36.