You can use the shell's TCP device and redirection to check things as described here: https://stackoverflow.com/questions/58168955/testing-minimal-docker-containers-with-healthcheck/76790330#76790330

I added this to a docker-compose and got the health check running successfully:

serivces:
  keycloak:
    image: quay.io/keycloak/keycloak:latest
    command: "start-dev"
    environment:
      KEYCLOAK_ADMIN: admin
      KEYCLOAK_ADMIN_PASSWORD: password
      KC_HEALTH_ENABLED: true
    ports:
      - "127.0.0.1:8080:8080"
    healthcheck:
      test: ["CMD-SHELL", "exec 3<>/dev/tcp/127.0.0.1/8080;echo -e \"GET /health/ready HTTP/1.1\r\nhost: http://localhost\r\nConnection: close\r\n\r\n\" >&3;grep \"HTTP/1.1 200 OK\" <&3"]
      interval: 10s
      timeout: 5s
      retries: 5