Change ENTRYPOINT to CMD for a debug tool

[rgroemmer]

/kind enhancement

Why this change?

I think it's more convenient to be able to overwrite the ENTRYPOINT of a debug tool instead of forcing it.
Now it will be able to have the same functionality as before, but with a single command instead of multiple.

Now it's also possible to exec one-shoot commands:

docker run --rm -it --name multitool multitool curl stackit.de

What has changed

• Changed in Dockerfile: ENTRYPOINT to CMD
• Updated Readme.md