Fix the Docker Compose to mount the config.toml volume as file and not as a directory

[timoa]

Fix an error on the docker-compose.yaml file since the config.toml volume mount does not specify the correct type.

It is mounting config.toml as a directory instead of a file:

docker compose up

[+] Running 2/0
 ✔ Container perplexica-searxng-1                             Running    0.0s                                                                                                                    
 ✔ Container perplexica-perplexica-frontend-1         Created   0.0s                                                                                                                           
Attaching to perplexica-backend-1, perplexica-frontend-1, searxng-1
Gracefully stopping... (press Ctrl+C again to force)
Error response from daemon: failed to create task for container: failed to create shim task: 
OCI runtime create failed: runc create failed: unable to start container process: error during container init: 
error mounting "/Users/***/***/Perplexica/config.toml" to rootfs at "/home/perplexica": 
mount /Users/***/***/Perplexica/config.toml:/home/perplexica (via /proc/self/fd/6), flags: 0x5000: 
not a directory: unknown: Are you trying to mount a directory onto a file (or vice-versa)? 
Check if the specified host path exists and is the expected type

[timoa]

I'm closing this PR since I made a new one with Dockerfile size optimization + security updates