Reintroduce `fetch()` method in ContainerController

It has been removed in commit f38fbfa373e132c0c0bd93ddcf7e658f13c6a142,
but it was still used, for instance, for the "top" action.
Merge pull request #33 from willdurand/fix-undefined-fetch-method

Reintroduce `fetch()` method in ContainerController