Added proper handling of incorrect values HIVED_UID to docker_entrypoint.sh

The docker_entrypoint.sh script should correctly handle incorrect values of HIVED_UID since there's a possibility that the Docker image will be started directly by docker run or from docker compose rather than using provided run_hived_img.sh.

Without the proposed changes, starting the container with HIVED_UID set to empty or non-integer value causes warning [: $HIVED_UID: integer expression expected to be displayed and the container continuing to run - and likely crashing later on.

With the proposed changes setting HIVED_UID to an empty value prints a warning and leaves the hived user with its default UID of 2000, while setting it to a non-integer value causes the container to crash with an appropriate error.