No suitable JDBC Driver when attempting to connect to Docker Postgres DB

I am working on a to-do list using a Java server and Postgres DB – specifically trying to set up a local dev environment using Docker Compose. The server has previously been deployed to Heroku and the database connection works without trouble. I am getting a suitable driver not found error when attempting to establish a connection between the server and DB in Docker.

The Java DB connection code:

    public void connectToDatabase() {
        try {
            String url = System.getenv("JDBC_DATABASE_URL");
            conn = DriverManager.getConnection(url);
            System.out.println("Database Connection Successful");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }

The Java Server Dockerfile:

FROM gradle:7.4-jdk17-alpine
ADD --chown=gradle . /code
WORKDIR /code
EXPOSE 5000
CMD ["gradle", "--stacktrace", "run"]

The image builds without problems. However, when starting with docker compose up, I get the following error: java.sql.SQLException: No suitable driver found for "jdbc:postgresql://tasks-db:5432/test-tasks-db?user=postgres&password=postgres"

The server still runs, just without the DB connection – I can access other endpoints/features.

Docker Compose:

version: "3.9"
services:
  java-service:
    build: 
      context: ./EchoServer/
      dockerfile: Dockerfile
    ports:
      - "5000:5000"
    environment:
      - PORT=5000
      - JDBC_DATABASE_URL="jdbc:postgresql://tasks-db:5432/test-tasks-db?user=postgres&password=postgres"
  tasks-db:
    image: postgres
    restart: always
    ports:
      - "1235:5432"
    environment:
      - POSTGRES_USER=postgres
      - POSTGRES_PASSWORD=postgres
    volumes: 
      - tasks-db:/var/lib/postgresql/data
volumes:
  tasks-db:
    driver: local
  logvolume01: {}

Grateful for any help, have been blocked on this most of the evening.

EDIT: build.gradle dependencies:

dependencies {
    testImplementation 'org.junit.jupiter:junit-jupiter-api:5.7.0'
    testRuntimeOnly 'org.junit.jupiter:junit-jupiter-engine:5.7.0'
    implementation 'org.json:json:20210307'
    implementation 'org.postgresql:postgresql:42.3.1'
}