What is Exited (255) code of docker container and how to overcome?

Hello All,

We are running custom docker container using docker-compose file with applied restart policy.
observation -
When we reboot system and docker start so container is not up and throw exited error(255).
How to ensure custom container are always running and up ?
Why container is not up and start at every boot .

(Note - i am using ubuntu core 16 as OS platform and
docker version

sudo docker version

Client:
Version: 18.09.9
API version: 1.39
Go version: go1.13.4
Git commit: 1752eb3
Built: Sat Nov 16 01:05:26 2019
OS/Arch: linux/amd64
Experimental: false

Server:
Engine:
Version: 18.09.9
API version: 1.39 (minimum version 1.12)
Go version: go1.13.4
Git commit: 9552f2b
Built: Sat Nov 16 01:07:48 2019
OS/Arch: linux/amd64
Experimental: false
)