Check for root user in entrypoint

[Cilenco]

Currently the container image is assuming that the container is started as root user. Running the image with the --user uid:gid option crashes the container with the message:

su-exec: setgroups(0): Operation not permitted

The entrypoint should check if the current user is root (e.g. [ $(id -u) = 0 ]) and only run su-exec if necessary.