You don't need a sidecar to stream logs of squid, that's anti-pattern, instead just tell squid to write logs to /dev/stdout, like this:

  logfile_rotate 0
  cache_log stdio:/dev/stdout
  access_log stdio:/dev/stdout
  cache_store_log stdio:/dev/stdout

Running squid in container is a bit tricky, since it is indeed an ancient piece of software, but I have managed to run it successfully before with squid configuration like this:

  max_filedescriptors 1048576
  pid_filename /dev/shm/squid.pid
  cache_effective_user squid
  cache_effective_group squid

and deployment has these set, - UID 31 is squid user inside of container

  securityContext:
  runAsUser: 31
  runAsGroup: 31
  fsGroup: 31
  command: ["sh","-c","squid -z && sleep 3s; squid -N"]