fix(cmd): move unix domain sockets to /var/run

[brentos]

Summary

Unix domain socket files located in the prefix directory cause problems when mounting that directory in containers. In addition, it is common practice to store socket files in /var/run which is cleaned up automatically on server restart.

Checklist

• The Pull Request has tests
• A changelog file has been created under changelog/unreleased/kong or skip-changelog label added on PR if changelog is unnecessary. README.md
• There is a user-facing docs PR against https://github.com/Kong/docs.konghq.com - PUT DOCS PR HERE

Full changelog

• [Implement ...]

Issue reference

Fix KAG-328

[CLAassistant]

All committers have signed the CLA.

[fffonion]

we will still need the logic, but instead removing dangling sockets in /var/run/kong directory.
I was wrong with my previous comment some days ago, it appears the /run or /var/run directory is not a volatile fs inside container; it's only volatile in a normal system. So the contents under this directory persists between container restarts.

[fffonion]

Please also consider use cases when user doesn't start Kong through systemd (e.g. a plain kong start). In such case, even the ReadWritePaths I mentioned above won't work because no party is going to create that directory. So does the systemd-tmpfiles approach after a system reboot.

In conclusion, we need to consider following use cases:

• container based
• systemd controlled services
• kong start
  All above use cases should work with fresh install, or after a system (container) restart.