ENTRYPOINT in Dockerfile

The ENTRYPOINT instruction in Dockerfile is like the CMD instruction as you can specify the program to be run then the container starts.

docker run ubuntu [COMMAND]

And everything you specify in [COMMAND] will get appended to ENTRYPOINT.
For example if you have ENTRYPOINT as sleep and [COMMAND] as 10:

docker run ubuntu 10

the command that will run will be:

sleep 10

To modify ENTRYPOINT during the runtime use --entrypoint switch for docker command:

docker run --entrypoint sleep2.0 ubuntu 10

The final command at statup will be:

sleep2.0 10