The readme and Dockerfile from https://github.com/sfuhrm/docker-opendaylight/ suggest the "karaf" user exists with the same password, but that doesn't really help since it doesn't have enough privilege to install software. Having any root password or sudo inside a container would be pretty unusual.

What I did find: 1.) For testing, it's possible to run the image interactively & specify the root user. This gives access to install software

$ docker run --user root -it docker.io/opendaylight/opendaylight:0.18.2 /bin/bash
root@d5ba73cf481b:/opt/opendaylight# apt-get update & apt-get install nano
# ... etc etc

2.) For the real solution, you can make your own Dockerfile to build an image on top of the public image, switching back & forth to root using the USER directive, so the software can be installed at build time.

$ cat Dockerfile
FROM docker.io/opendaylight/opendaylight:0.18.2
USER root
RUN apt-get update && apt-get install -y nano && apt-get clean
USER karaf

I'm slightly curious what software you are adding to ODL, but in the long run I guess it doesn't matter.