fix: Improve docker context support

[achimnol]

Also a follow-up to #1694.
Overall, this PR improves the search process of docker socket locations and make it consistent across other Backend.AI codebase and the installer.

• Share the implementation of common.docker.get_docker_connector() with the installer and other Backend.AI codebase.
• Update get_docker_connector() to support docker contexts (~/.docker/contexts/meta/...) to detect the currently used docker host settings.
  • According to docker context ls output, I assumed that meta.json always has .Endpoints.docker.Host field.
• Clarify the priority of docker host configurations as shown in the precedence order:
  • DOCKER_HOST environment variable
  • The current client-side docker context
  • The platform-specific list of potential docker socket locations
• Fix the installer:
  • Correct the usage of determine_docker_sudo()
  • Let it skip sudo chmod 666 <docker-socket> if the docker socket file resides inside the user's home directory (e.g., OrbStack, Docker Desktop) instead of the system-global /run or /var/run.

Example on macOS with OrbStack

$ docker context ls
NAME         DESCRIPTION                               DOCKER ENDPOINT                                  ERROR
default      Current DOCKER_HOST based configuration   unix:///var/run/docker.sock
orbstack *   OrbStack                                  unix:///Users/joongi/.orbstack/run/docker.sock

>>> from ai.backend.common.docker import get_docker_context_host, get_docker_connector
>>> get_docker_context_host()
'unix:///Users/joongi/.orbstack/run/docker.sock'
>>> get_docker_connector()
(PosixPath('/Users/joongi/.orbstack/run/docker.sock'), URL('http://localhost'), <aiohttp.connector.UnixConnector object at 0x104663ed0>)

Example on Linux with native Docker installation

>>> from ai.backend.common.docker import get_docker_context_host, get_docker_connector
>>> get_docker_context_host()
>>> get_docker_connector()
(PosixPath('/run/docker.sock'), URL('http://localhost'), <aiohttp.connector.UnixConnector object at 0xffffab584950>)

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Test case(s) to demonstrate the difference of before/after