fix(compose): stop pointing at the tag the build dropped

These five stacks still asked for :latest, but the build stopped pushing
it - on main it only pushes main and prod, on dev only dev. Every one of
these images therefore resolved only because the registry still had a
stale :latest from before that change, and the next time one of them was
actually built the reference would have dangled.

Named for dtek-notif: of the seven, only that one still resolved at
:prod, because it is the only image not rebuilt since the build dropped
:latest - and it is the only one of these five stacks the deploy does not
manage (no `active` marker, and its file is docker-compose.yaml, which
the COMPOSE_STACKS glob does not even match). Touching
dtek_notif/docker-compose.yaml matches dtek_notif/* in the build's
changed-service detector, so pushing this builds it and publishes
:prod for it too.

Verified the other six resolve at :prod in the registry.

userbot/compose.yaml

@@ -3,7 +3,7 @@ services:
     build:
       context: .
       dockerfile: Dockerfile
-    image: gcr.forust.xyz/forust/userbot:latest
+    image: gcr.forust.xyz/forust/userbot:prod
     pull_policy: build
     restart: unless-stopped
     env_file:
@@ -18,7 +18,7 @@ services:
       - 1.1.1.1
 
   anna:
-    image: gcr.forust.xyz/forust/userbot:latest
+    image: gcr.forust.xyz/forust/userbot:prod
     pull_policy: build
     depends_on:
       - forust