Use ENTRYPOINT instead of CMD in Dockerfile

so that we can run, for example,

> docker run --rm -it kaf nodes

instead of

> docker run --rm -it kaf kaf nodes

Besides, we build image from scratch, so we have no other commands to
run from the image.

Dockerfile

@@ -20,4 +20,4 @@ COPY --from=BuildStage /kaf /bin/kaf
 USER 1001
 
 # Run
-CMD ["/bin/kaf"]
+ENTRYPOINT ["/bin/kaf"]