Fix alterContainerL

Fix alterContainerL

The original version accidentally used `Map.update` instead of
`Map.alter`, which broke insertions (the function would only have
an effect if the key was already in the container).

Signed-off-by: Niklas Hambuechen <niklash@google.com>
Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>