Allow user to specify a docker image using compose

This makes it easier to test local changes. Usage is basically:

$ docker build -t whatever:latest .
$ IMAGE=whatever:latest docker compose up -d

docker-compose.yml

@@ -1,7 +1,7 @@
 version: '3.8'
 services:
   alltalk-tts:
-    image: alltalk:v1-9c
+    image: ${IMAGE:-alltalk:v1-9c}
     restart: unless-stopped
     ports:
       - "7851:7851"