New environmental variable to check the service ports of a compute session

[adrysn]

Currently, there is no way for a user to check the pre-open ports inside a container. So, let's add a new environment variable, such as BACKENDAI_SERVICE_PORTS by reading the image's label, service-ports. It may be appended under the following code block: https://github.com/lablup/backend.ai/blob/39c292eff9ff0978e8371d57349f298516335f99/src/ai/backend/manager/registry.py#L1235-L1242.