Make docker-compose.yml deployable as-is on the production server

Adds image: alongside build: so `docker compose build && push` works
locally, while the server can just `docker compose pull && up -d`
without needing the Dockerfile/build context. .env.example now
documents each variable and adds HOST_PORT.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docker-compose.yml

@@ -1,8 +1,13 @@
 services:
   honey-links:
+    # image: used when pulling on the server (docker compose pull && up -d).
+    # build: lets `docker compose build` / `docker compose push` create and
+    #        publish that same image locally, instead of manual docker
+    #        build/tag/push commands.
+    image: gitea.honeyy.duckdns.org/website/honey-links:latest
     build: .
     ports:
-      - "8080:80"
+      - "${HOST_PORT:-8080}:80"
     environment:
       - ADMIN_PASSWORD=${ADMIN_PASSWORD}
       - SECRET_KEY=${SECRET_KEY}