added sample .env

.env.example

@@ -0,0 +1,23 @@
+##################
+## Frontend env ##
+##################
+WEB_PORT=3000
+PUBLIC_BASE_PATH=/watch-party/
+BACKEND_ORIGIN=http://api:8082
+
+#################
+## Backend env ##
+#################
+# Postgres (container)
+POSTGRES_DB=watchparty
+POSTGRES_USER=admin
+POSTGRES_PASSWORD=admin
+TZ=Asia/Tokyo
+COMPOSE_PLATFORM=linux/arm64/v8
+
+# App runtime (inside Docker network)
+PGHOST=db
+POSTGRES_PORT=5432
+PGSSLMODE=disable
+GIN_MODE=release
+ADDR=:8082