API changes
- `containers-list` no longer lists containers from subdatasets by
default. Specify `--recursive` to do so.
- `containers-run` no longer considers subdataset containers in its
automatic selection of a container name when no name is specified.
If the current dataset has one container, that container is
selected. Subdataset containers must always be explicitly
specified.
New features
- `containers-add` learned to update a previous container when passed
`--update`.
- `containers-add` now supports Singularity's "docker://" scheme in
the URL.
- To avoid unnecessary recursion into subdatasets, `containers-run`
now decides to look for containers in subdatasets based on whether
the name has a slash (which is true of all subdataset containers).