Add Docker support and update Java version to 21

DOCKER.md

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+# How to run container
+
+### Build
+In the root directory of project there is `Dockerfile` in which image is defined.
+<br> Build the image:
+```bash
+docker build -t listhub:1.0 .
+```
+Then after building the image you can run it. You will need to define some environment variables.
+```bash
+docker run -d \
+--name listhub_backend \
+-p 8080:8080 \
+-e JWT_SECRET=<paste_your_secret_here> \
+-e DB_URL=jdbc:postgresql://db:5432/postgres \
+-e DB_USER=postgres \
+-e DB_PASS=postgres \
+listhub_backend:1.0
+```
+
+or you can use compose file with .env file
+
+```text
+// .env
+JWT_SECRET=<paste_your_secret_here>
+
+DB_URL=jdbc:postgresql://db:5432/postgres
+DB_USER=listhub
+DB_PASS=postgres
+```
+
+```yaml
+# docker-compose.yaml
+services:
+  db:
+    image: postgres:14
+    restart: unless-stopped
+    environment:
+      POSTGRES_DB: listhub
+      POSTGRES_USER: postgres
+      POSTGRES_PASSWORD: postgres
+    ports:
+      - "5432:5432"
+    volumes:
+      - postgres_data:/var/lib/postgresql/data
+
+  listhub_backend:
+    build: .
+    restart: unless-stopped
+    depends_on:
+      - db
+    environment:
+      DB_URL: jdbc:postgresql://db:5432/listhub
+      DB_USER: postgres
+      DB_PASS: postgres
+      JWT_SECRET: awdpokawodjawoidjawidjoij120391829d3j8a0jd8s9dawd
+    ports:
+      - "8080:8080"
+    volumes:
+      - listhub_data:/app
+
+volumes:
+  postgres_data:
+  listhub_data:
+```