the image agrees with itself about the timezone (V-545)

compose set TZ=Europe/Samara and Go read it, so clock replies and quiet
hours were already local. But /etc/localtime in the image pointed at
Etc/UTC, so a caller asking the system zone instead of the environment
answered UTC. The reminder path shells out to python dateparser, which is
such a caller.

TZ is now a build arg on the runtime stage. It points the symlink, writes
/etc/timezone and sets ENV TZ, so the image is local on its own. Compose
passes the zone it already declares, so the zone stays written in one
place.

docker-compose.yml

@@ -12,7 +12,12 @@ x-image: &image
   # build on EVERY service (same image name ⇒ built once) so `docker compose
   # build <anyservice>` actually rebuilds. With build on only one service, the
   # others silently no-op and you deploy a stale binary.
-  build: .
+  build:
+    context: .
+    # the zone is declared once, here. The image points /etc/localtime at it so
+    # a caller reading the system zone agrees with one reading TZ (V-545).
+    args:
+      TZ: Europe/Samara
   pull_policy: never   # only ever the locally-built image
   restart: unless-stopped
   # local time for clock/date replies AND quiet-hours evaluation. Change to