How to edit file in a docker container?

As in the comments, there’s no default editor set - strange - $EDITOR env variable is empty. You can login into container with:

docker exec -it <container> bash and run:

apt-get update
apt-get install vim

or use the following Dockerfile:

FROM  confluent/postgres-bw:0.1

RUN ["apt-get", "update"]
RUN ["apt-get", "install", "-y", "vim"]
EDIT

Docker images are delivered trimmed to bare minimum - so no editor is installed with the shipped container. That’s why there’s a need to install it manually.