logs: select manager container when multiple exist

This sets the container to `manager` which is used by all Flux
controllers as the container name.

The other options I thought about were selecting the first, or doing
something with image detection. But both can be sensitive to either
users adding their patch as a first entry, or e.g. mirroring the image
to a different name.

Signed-off-by: Hidde Beydals <hello@hidde.co>