It could be permission problem. check if root user can access that file. if yes, check a regular user see if can access, if not, it is a permission problem.
Also, if you use CageFS, perform a force update to make sure that file is available in the user's cage.
LB follow standard cache-control for caching static file.
It can be configured to turn on cache based on file suffix used in the URL.
Caching static file in memory is expensive, our cache is file backend, store cache to a SSD disk should be good.
If each Docker container needs its own web server, it is not going to work, if web server is shared for all the docker containers, one license is fine.
you need to fix the 503 error, it is due to PHP error, has nothing to do with the web server.
Please follow this to figure out the root cause of 503 error.
https://www.litespeedtech.com/support/wiki/doku.php/litespeed_wiki:php:503-errors
it is triggered by a mod_security rule "drop" action, you should review your mod_security audit log to find out which rule was triggered, then disable that rule.