📝 added README

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README.md

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+# Billy Gin 🍶
+
+Seamless blue/green deployment based on Nginx Proxy Manager.
+
+## Blue/Green deployment
+
+🤔 How to roll a new version of an app without any
+downtime? **Blue/Green deployment** is a deployment
+workflow where you have 2 independent app containers.
+
+💭 Imagine you have an API `api.example.com`. With
+Blue/Green workflow you start with the following
+configuration:
+
+| 🟢              | 🔵                    |
+|-----------------|-----------------------|
+| api.example.com | api-stage.example.com |
+| `v1.0.0`        | `v1.0.0`              |
+| Container `A`   | Container `B`         |
+
+Your production uses `api.example.com` so it
+must have 0 downtime. Otherwise, `api-stage.example.com`
+is only for internal usage, so it doesn't matter
+if there is any downtime.
+
+You roll out an update (`v1.0.1`) to the container `B`.
+If it's required, it's safe to restart it, shut down etc.
+Your current configuration looks like this:
+
+| 🟢              | 🔵                    |
+|-----------------|-----------------------|
+| api.example.com | api-stage.example.com |
+| `v1.0.0`        | `v1.0.1`              |
+| Container `A`   | Container `B`         |
+
+After you spin up and tested a new version
+(remember, you can still access it via `api-stage.example.com`) just swap gateways of Green and Blue
+hosts:
+
+| 🟢              | 🔵                    |
+|-----------------|-----------------------|
+| api.example.com | api-stage.example.com |
+| `v1.0.1`        | `v1.0.0`              |
+| Container `B`   | Container `A`         |
+
+
+🎉 Done. You have deployed a new udpate with 0 downtime.
+
+## Usage
+
+Do a `POST /toggle` request with following body:
+```json
+{
+  "secret": "supersecret"
+}
+```
+That's it. You have seamlessly swapped two
+containers.
+
+## Setup
+
+**❗️ Before start:**
+
+This is an "add-on" for the [Nginx Proxy Manager](https://github.com/NginxProxyManager/nginx-proxy-manager). If you do not use it this project isn't suitable for you.
+
+I really recommend you to see the original project. It's a handy tool which makes DevOps much easier ✨
+
+### 📝 Service account
+
+BillyGin needs a **service account** to work.
+You can create it under **Users > Add users** in the
+Nginx Proxy Manager UI.
+
+If you don't want to give admin rights, select `Manage`
+role for all **Proxy hosts**.
+
+Then, set a password for the created user (_three dots, Change password_).
+
+### 🟢🔵 Green and Blue containers
+
+To make possible for BillyGin to operate with proxy hosts
+it's required to create those hosts first.
+
+Do it as usual under **Proxy hosts**. Create records
+for *green* and *blue* hosts.
+
+Example:
+
+- Green host:
+  - Domain: `api.example.com`
+  - Forward hostname: `api_a`
+
+- Blue host:
+  - Domain: `api-stage.example.com`
+  - Forward hostname: `api_b`
+
+
+### 🌲 Environment variables
+
+BillyGin is using environment variables to configure
+itself. You can pass them via `.env` file.
+
+```env
+PORT=3000
+HOST="0.0.0.0"
+
+NGINX_PROXY_MANAGER_URL=http://proxy:81
+NGINX_PROXY_MANAGER_SA_EMAIL="[email protected]"
+NGINX_PROXY_MANAGER_SA_SECRET="somesecret"
+
+GREEN_DOMAIN=api.example.com
+BLUE_DOMAIN=api-stage.example.com
+
+A_FORWARD_HOST=api_a
+B_FORWARD_HOST=api_b
+
+SECRET="anothersecret"
+```
+
+- `NGINX_PROXY_MANAGER_URL` - instance of Nginx Proxy
+  Manager
+- `NGINX_PROXY_MANAGER_SA_EMAIL` - email of created
+  service account (user)
+- `NGINX_PROXY_MANAGER_SA_SECRET` - password of
+  created service account
+- `GREEN_DOMAIN` - "production" domain (without
+  protocol or slashes)
+- `BLUE_DOMAIN` - "staging" domain (same rules)
+- `A_FORWARD_HOST` - `A` container host (without port, protocol]
+  or slashes)
+- `B_FORWARD_HOST` - `B` container host (same rules)
+- `SECRET` - service key to sign 
+requests (any string you want)
+
+**All listed variables are required.**
+
+
+### 🖥️ Run natively
+**Prerequisites**
+
+- 🚀 NodeJS v18
+
+Clone this repo. Create `.env` file with
+configurations. Run following commands:
+
+```console
+npm i
+npm run build
+npm start
+```
+
+### 🐳 Docker compose
+**Prerequisites**
+
+- 🐳 Docker (compose)
+
+Create a `docker-compose.yml` file with
+the following contents:
+
+```yaml
+version: "3"
+services:
+  billy-gin:
+    container_name: billy-gin
+    image: "mrfullset/billy-gin:latest"
+    env_file:
+      - .env
+```
+
+Put `.env` file near to `docker-compose.yml`. Run:
+
+```console
+docker compose up -d
+```
+
+### ☝️ Recomendations
+
+If you use a proxy host to Nginx Proxy Manager
+itself, it's a good idea to add BillyGin as
+`Custom location` to its record, so you don't need
+to register a separate domain for it or expose ports
+from Docker. Example settings:
+
+- Location: `/billy-gin/`
+- Forward hostname: `billy-gin/`.
+
+**Preserve slashes as they are in the example**
+
+
+