docs(dev): update README.md with instructions for using single-compose.yml (#676)

feat(single-compose.yml): add single-compose.yml for building leaner app container without meilisearch and mongodb services
- This is useful for deploying on Google, Azure, etc., as a single, leaner container.
- Instructions for running the container are added to the README.md file.
- The container requires a MongoDB Atlas connection string for the `MONGO_URI` environment variable.
- Remote Meilisearch may also be possible, but is not tested.

docs/dev/single-compose.yml

@@ -0,0 +1,31 @@
+version: "3.4"
+
+services:
+  api:
+    container_name: LibreChat-App
+    ports:
+      - 3080:3080               # Change it to 9000:3080 to use nginx
+    image: librechat                # Comment this & uncomment below to build from docker hub image
+    build:                                   # ^------
+      context: ../../                         # ^------
+      target: node                             # ^------v
+    # image: ghcr.io/danny-avila/librechat:latest # Uncomment this & comment above to build from docker hub image
+    restart: always
+    extra_hosts: # if you are running APIs on docker you need access to, you will need to uncomment this line and next
+    - "host.docker.internal:host-gateway"
+    env_file:
+      - ../../.env
+    environment:
+      - HOST=0.0.0.0
+      # it's best to set it in your .env file, but uncomment if you prefer it in compose file
+      # MONGO_URI=<Your MongoDB Atlas connection string here> 
+      # - CHATGPT_REVERSE_PROXY=http://host.docker.internal:8080/api/conversation # if you are hosting your own chatgpt reverse proxy with docker
+      # - OPENAI_REVERSE_PROXY=http://host.docker.internal:8070/v1/chat/completions # if you are hosting your own chatgpt reverse proxy with docker
+    volumes:
+      - /app/client/node_modules
+      - ../../api:/app/api
+      - ../../.env:/app/.env
+      - ../../.env.development:/app/.env.development
+      - ../../.env.production:/app/.env.production
+      - /app/api/node_modules
+      - ../../images:/app/client/public/images