feature request: support running admin app independently

[Shiti]

What problem did you meet?

I ran into an issue cos I was trying to run logto on Google cloud run which requires setting the environment variable PORT to be used by the load balancer. Only one port can be configured per service. So, I started logto as 2 separate jobs - one for the core app and one for admin.
The issue was that for admin service, the core app still runs and uses the environment variable PORT. For load balancing to work correctly for my admin instance, the PORT environment variable's value should be that of ADMIN_PORT.

Describe what you'd like Logto to have

I can think of two ways that could help:

1. Allow disabling core app for the admin only instance. It would rely on the ENDPOINT url to co-ordinate with the core app.
2. Change environment variable name used for configuring port.

I worked around this was by publishing a custom image - using APP_PORT and ADMIN_APP_PORT instead of PORT and ADMIN_PORT