Add `exec` to the entry points of Docker files in the project

[mir-am]

Description

This is a tiny PR to add exec to ENTRYPOINT of Docker files of both the FASTEN server and REST API. This is recommended in the official Docker documentation here.

Motivation and context

Currently, Docker files use the shell form for ENTRYPOINT, which prevents the Docker container to receive Unix signals. This PR addresses this issue.

Testing

Not yet.