misc: use the "unstable" tag between releases

In addition the "latest" tag will be gone with the next stable release.

Going forward this is our versioning scheme:

- stable-XXXX: stable release
- unstable: daily unstable rolling release
- unstable-XXXX-YY: daily unstable release

docker-compose.yml

@@ -3,7 +3,7 @@ version: '3'
 services:
     # Frontend
     web:
-        image: jitsi/web:latest
+        image: jitsi/web:unstable
         restart: ${RESTART_POLICY}
         ports:
             - '${HTTP_PORT}:80'
@@ -139,7 +139,7 @@ services:
 
     # XMPP server
     prosody:
-        image: jitsi/prosody:latest
+        image: jitsi/prosody:unstable
         restart: ${RESTART_POLICY}
         expose:
             - '${XMPP_PORT:-5222}'
@@ -221,7 +221,7 @@ services:
 
     # Focus component
     jicofo:
-        image: jitsi/jicofo:latest
+        image: jitsi/jicofo:unstable
         restart: ${RESTART_POLICY}
         volumes:
             - ${CONFIG}/jicofo:/config:Z
@@ -273,7 +273,7 @@ services:
 
     # Video bridge
     jvb:
-        image: jitsi/jvb:latest
+        image: jitsi/jvb:unstable
         restart: ${RESTART_POLICY}
         ports:
             - '${JVB_PORT:-10000}:${JVB_PORT:-10000}/udp'