To keep a container running when you start it with docker-compose, use the following command
command: tail -F anything
In the above command the last part anything should be included literally, and the assumption is that such a file is not present in the container, but with the -F option (capital -F not to be confused with -f which in contrast will terminate immediateley if the file is not found) the tail command will wait forever for the file anything to appear. A forever waiting process is basically what we need.
So your docker-compose.yml becomes
version: '2'
services:
my-test:
image: ubuntu
command: tail -F anything
and you can run a shell to get into the container using the following command
docker exec -i -t composename_my-test_1 bash
where composename is the name that docker-compose prepends to your containers.