Use CANASTA_IMAGE environment variable for web service image (#73)

Change the web service image from a hardcoded registry reference to use
an environment variable with a default fallback:

  image: ${CANASTA_IMAGE:-ghcr.io/canastawiki/canasta:latest}

This allows the Canasta CLI to override the image when building from
local source with the --build-from flag. The CLI sets CANASTA_IMAGE
in the .env file to point to the locally built image (e.g., canasta:local).

When CANASTA_IMAGE is not set, the default behavior remains unchanged,
pulling from the GitHub Container Registry.

Author: cicalese

docker-compose.yml

@@ -22,7 +22,7 @@ services:
       - ./my.cnf:/etc/my.cnf
 
   web:
-    image: ghcr.io/canastawiki/canasta:latest
+    image: ${CANASTA_IMAGE:-ghcr.io/canastawiki/canasta:latest}
     restart: unless-stopped
     extra_hosts:
       - "gateway.docker.internal:host-gateway"