compose: add ability to override image versions

Useful for local development, overriding to latest is possible now
without changing the compose file itself.

docker-compose.yml

@@ -3,8 +3,8 @@ version: '3'
 services:
     # Frontend
     web:
-        image: jitsi/web:unstable
-        restart: ${RESTART_POLICY}
+        image: jitsi/web:${JITSI_IMAGE_VERSION:-unstable}
+        restart: ${RESTART_POLICY:-unless-stopped}
         ports:
             - '${HTTP_PORT}:80'
             - '${HTTPS_PORT}:443'
@@ -140,8 +140,8 @@ services:
 
     # XMPP server
     prosody:
-        image: jitsi/prosody:unstable
-        restart: ${RESTART_POLICY}
+        image: jitsi/prosody:${JITSI_IMAGE_VERSION:-unstable}
+        restart: ${RESTART_POLICY:-unless-stopped}
         expose:
             - '${XMPP_PORT:-5222}'
             - '5347'
@@ -225,8 +225,8 @@ services:
 
     # Focus component
     jicofo:
-        image: jitsi/jicofo:unstable
-        restart: ${RESTART_POLICY}
+        image: jitsi/jicofo:${JITSI_IMAGE_VERSION:-unstable}
+        restart: ${RESTART_POLICY:-unless-stopped}
         volumes:
             - ${CONFIG}/jicofo:/config:Z
         environment:
@@ -275,8 +275,8 @@ services:
 
     # Video bridge
     jvb:
-        image: jitsi/jvb:unstable
-        restart: ${RESTART_POLICY}
+        image: jitsi/jvb:${JITSI_IMAGE_VERSION:-unstable}
+        restart: ${RESTART_POLICY:-unless-stopped}
         ports:
             - '${JVB_PORT:-10000}:${JVB_PORT:-10000}/udp'
             - '127.0.0.1:${JVB_COLIBRI_PORT:-8080}:8080'