Use ENTRYPOINT instead of CMD

[Tim Besard]

It is convenient to be able to treat docker run --rm -it julia just as you would use the julia binary. This is currently not possible, because the containers here set CMD and not ENTRYPOINT:

❯ docker run --rm -it julia -e '42'
docker: Error response from daemon: failed to create shim task: OCI runtime create failed: runc create failed: unable to start container process: exec: "-e": executable file not found in $PATH: unknown.

[Tianon Gravi]

Unfortunately, we can't just swap to ENTRYPOINT as-is (https://github.com/docker-library/official-images#consistency), but we can write a script that fixes the problem -- thanks for raising it! (I'll get to work on a PR ❤️)

[Tim Besard]

Ah, interesting! Thanks for the PR, the entrypoint script should cover most common cases.