make entrypoint executable

[phoehnel]

When trying to build & run the Dockerfile in it's current version, I came across the problem, that the entrypoint was not executable.

This results in the error:

docker: Error response from daemon: failed to create task for container: failed to create shim task: OCI runtime create failed: runc create failed: unable to start container process: exec: "/app/entrypoint.sh": permission denied: unknown.

I am unsure if this ever worked in other environments, or if something like the default-mode changed in the base image.
Nevertheless, this change ensures the entrypoint is copied with the right permissions.