Enable the custom entrypoint script support added in #1964 to be run as root.

[AkashiSN]

In #1964, the scripts were executed as $user by run_as, but for example, new packages could not be added with apt because they lacked root privileges.

Therefore, I have made changes to ensure that the custom scripts are executed with root privileges.

Also, as pointed out in the comments at #r1191243057, I have made sure that the custom scripts are executed with /bin/bash, even when they do not have execution permissions.

For example, when using a custom script in a k8s configmap, it is not possible to assign execution permissions, and chmod cannot be used because it is mounted as ReadOnly. There are cases where it is difficult to assign execution permissions, even when you want to add a custom script.

Ideally, I wanted to do . script(sourcing) like in docker-library/postgres#452, but since entrypoint.sh is executed with /bin/sh, I decided to launch a new /bin/bash shell and execute it for the sake of extensibility.

Additionally, for cases where you want to execute as the nextcloud user, I've made it so that you can reference:

user=$user /bin/bash "${script_file_path}"

inside the custom script with:

run_as() {
  su -p $user -s /bin/sh -c "$1"
}

My apologies for any awkward English as I am not a native speaker.

Best Regards.

[AkashiSN]

As per #1964 (comment), this hook script is not intended to install packages and is running on www-data, so close this PR.