Making changes in a running docker container

In this example I assume that you are using a tomcat contanier and a webapp deloyed in the container.
Problem: 
I have a webapp running in a docker container and I want to change the log level.
Solution:
Get a bash shell into your container.
docker exec -it containerid /bin/bash Do the change that you need to do.

Stop your container
docker stop containerid Start your container
docker start containerid This change will not get saved into the image as soon as you destory your container instance you will loose the change.