how to run single node docker swarm and run pytest in github actions

Question

I am using just a single node for my project. But I still want to use docker swarm mode, to learn and for possible scaling later.

I want to write a github action workflow file that will create such a swarm, run pytest on the container, and if tests pass, push code to production server.

On my development local machine, following commands work:

docker swarm init
docker stack deploy -c docker-compose.yml ex
docker exec $(docker ps -q -f name=ex_backend) pytest

Corresponding github action workflow do not work though:

  - run: docker swarm init
  - run: docker stack deploy -c docker-compose.yml ex
  - run: docker exec $(docker ps -q -f name=ex_backend) pytest

I get the error "docker exec" requires at least 2 arguments.

Upon inspection I notice that docker ps -q -f name=ex_backend does not return anything, hence the last command fails.

- run: docker service ps ex_backend shows:

ID             NAME           IMAGE                             NODE          DESIRED STATE   CURRENT STATE                      ERROR     PORTS
ppfwcobs1x33   ex_backend.1   xxx/ex_backend:0.0.1   fv-az36-929   Running         Preparing less than a second ago    

- run: docker service ls shows following:

ID             NAME          MODE         REPLICAS   IMAGE                             PORTS
hzp5gigrmjm4   ex_backend    replicated   0/1        xxx/ex_backend:0.0.1   *:8888->8888/tcp
iuaparyt8qws   ex_postgres   replicated   0/1        postgres:12                       *:5432->5432/tcp

In my local machine replicas column value is '1/1'. Here '0/1'.

So, does that mean container is not started? I am new to docker swarm and github actions and stuck here at this point. How can I run pytest in the ex_backend service?

EDIT: here is the full workflow file(.github/workflows/build.yml):

name: Build and Deploy
on:
  push:
    branches:
      - "main"
jobs:
  build:
    runs-on: ubuntu-20.04
    steps:
      - name: Get the code from github
        uses: actions/checkout@v2
      - name: Login to Docker Hub
        uses: docker/login-action@v1
        with:
          username: ${{ secrets.DOCKER_HUB_USERNAME }}
          password: ${{ secrets.DOCKER_HUB_ACCESS_TOKEN }} 
      - run: docker swarm init
      - run: docker stack deploy -c docker-compose.yml exs --with-registry-auth
      - run: docker exec $(docker ps -q -f name=exs_backend) pytest -x

And docker-compose.yml:

version: "3.7"
services:
  backend:
    image: xxx/exi_backend:latest
    ports:
      - 8888:8888
    tty: true
    volumes:
      - ./backend:/app/:cached
      - ./.docker/.ipython:/root/.ipython:cached
    environment:
      PYTHONPATH: .
      DATABASE_URL: "postgresql://postgres:postgres123@postgres:5432/postgres"
    depends_on:
      - "postgres"
    command: bash -c "uvicorn app.main:app --host 0.0.0.0 --port 8888 --reload"
  postgres:
    image: postgres:12
    restart: always
    environment:
      POSTGRES_USER: postgres
      POSTGRES_PASSWORD: postgres123
    ports:
      - "5432:5432"
    volumes:
      - db-data:/var/lib/postgresql/data:cached
volumes:
  db-data: