build(backend): one shared Dockerfile with cargo cache mounts, sequential compose build, stripped release binaries

All four services now share one builder stage (dependencies compile once per
deploy, not once per service). BuildKit cache mounts keep the cargo registry
and target/ between deploys: a one-line change rebuilds in ~16 s instead of
recompiling the whole dependency tree.

backend/docker-compose.prod.yml

@@ -62,7 +62,8 @@ services:
   accounts-service:
     build:
       context: .
-      dockerfile: accounts-service/Dockerfile
+      dockerfile: Dockerfile
+      target: accounts-service
     restart: unless-stopped
     depends_on:
       postgres:
@@ -94,7 +95,8 @@ services:
   configs-service:
     build:
       context: .
-      dockerfile: configs-service/Dockerfile
+      dockerfile: Dockerfile
+      target: configs-service
     restart: unless-stopped
     depends_on:
       postgres:
@@ -111,7 +113,8 @@ services:
   chat-service:
     build:
       context: .
-      dockerfile: chat-service/Dockerfile
+      dockerfile: Dockerfile
+      target: chat-service
     restart: unless-stopped
     env_file: .env
     environment:
@@ -121,7 +124,8 @@ services:
   gateway:
     build:
       context: .
-      dockerfile: gateway/Dockerfile
+      dockerfile: Dockerfile
+      target: gateway
     restart: unless-stopped
     depends_on:
       accounts-service: