docker: Align compose healthcheck with admin port

The compose healthcheck probes http://127.0.0.1:81/api, but the binary
defaults ADMIN_PORT to 8081, so a stock compose up never becomes healthy.

Set ADMIN_PORT, PROXY_HTTP_PORT, and PROXY_HTTPS_PORT in compose so the
process listens on the Dockerfile EXPOSE ports. Document real binary
defaults in the README and keep the Docker section on 80/81/443 via env.

Link: #10

docker-compose.yml

@@ -12,14 +12,18 @@ services:
       - ./data:/app/data
     # user: "0:0"   # required when using PUID/PGID != built-in to allow binary to chown+drop
     # working_dir: /app   # binary uses CWD for relative data/ + data/www/
-    # environment:
-    #   - JWT_SECRET=  # optional; otherwise a random secret is stored in data/.jwt_secret
-    #   - DATA_DIR=/app/data
-    #   - WWW_DIR=/app/data/www
-    #   - PUID=1000
-    #   - PGID=1000
-    #   - DISABLE_IPV6=1
-    #   # PUID_NO_DROP=1   # if using low ports (80/443) + PUID: chown as root but skip drop (stay root for bind; default drop runs as PUID after, requires high ports or NET_BIND_SERVICE cap)
+    environment:
+      - ADMIN_PORT=81
+      - PROXY_HTTP_PORT=80
+      - PROXY_HTTPS_PORT=443
+      # ADMIN_HOST defaults to 127.0.0.1 (healthcheck hits 127.0.0.1:81)
+      # - JWT_SECRET=  # optional; otherwise a random secret is stored in data/.jwt_secret
+      # - DATA_DIR=/app/data
+      # - WWW_DIR=/app/data/www
+      # - PUID=1000
+      # - PGID=1000
+      # - DISABLE_IPV6=1
+      # # PUID_NO_DROP=1   # if using low ports (80/443) + PUID: chown as root but skip drop (stay root for bind; default drop runs as PUID after, requires high ports or NET_BIND_SERVICE cap)
     # To use optional SQL backend instead of default yaml:
     #   - DB_TYPE=postgres
     #   - DB_POSTGRES_HOST=db