Add compose env template

.env.docker.example

@@ -0,0 +1,26 @@
+# Copy to .env beside docker-compose.yml and fill in.
+#   cp .env.docker.example .env
+
+# --- required ---------------------------------------------------------------
+# Generate with: python -c "import secrets; print(secrets.token_hex(32))"
+SECRET_KEY=
+# Any long random string; only the db and app containers ever see it.
+POSTGRES_PASSWORD=
+
+# --- database ---------------------------------------------------------------
+POSTGRES_DB=lim
+POSTGRES_USER=lim
+
+# --- app --------------------------------------------------------------------
+# Host port the web UI is published on.
+LIM_PORT=8000
+# Backup archives to retain after a UI-initiated backup (0 = keep all).
+BACKUP_KEEP=7
+# Max upload size in bytes (default 50MB).
+MAX_CONTENT_LENGTH=52428800
+SETS_PER_PAGE=20
+
+# --- Brickset (optional) ----------------------------------------------------
+BRICKSET_API_KEY=
+BRICKSET_USERNAME=
+BRICKSET_PASSWORD=