Cannot start dockerized rails application when pulling it from Docker Hub

I dockerized a rails application which runs perfectly on my local machine. I configured my Github Actions to push the application into a private image on Docker Hub and when I try to pull and run it locally or on another machine, it pulls the image but the container then exits because Rails fails with the following error:

app  | rm: cannot remove 'tmp/pids/server.pid': No such file or directory
app  | => Booting Puma
app  | => Rails 6.0.3.3 application starting in development 
app  | => Run `rails server --help` for more startup options
app  | Exiting
app  | (erb):9:in `<main>': Cannot load database configuration: (NoMethodError)
app  | undefined method `[]' for nil:NilClass
app  |   from /usr/local/lib/ruby/2.7.0/erb.rb:905:in `eval'
app  |   from /usr/local/lib/ruby/2.7.0/erb.rb:905:in `result'
app  |   from /usr/local/bundle/gems/railties-6.0.3.3/lib/rails/application/configuration.rb:228:in `database_configuration'

I commented out the rm command and also both bin/rails commands to see if I can get the container to stay up and debug in the console but I had no chance.

From searching other SO questions I suppose there must be something wrong with my ENV or the database.yml in general but I don't understand why it runs perfectly when I don't pull the image from Docker Hub. Shouldn't the credentials be resolved either way? There are credentials for every environment in separate credential files.

Here is my code:

docker-compose.yml

version: '3'

volumes:
  db_data:
    driver: local
  app_data:
    driver: local

services:
  # database container
  postgres:
    image: postgres
    volumes:
      - app_data:/var/lib/postgresql/data
    ports:
      - 5439:5432
    environment:
      POSTGRES_DB: $DATABASE_NAME
      POSTGRES_USER: $DATABASE_USER
      POSTGRES_PASSWORD: $DATABASE_PASSWORD
    command: ["postgres", "-c", "log_statement=all"]
    restart: always

  backend:
    build: ./server
    volumes:
      - ./server:/var/workdir
    ports:
      - "3009:3000"
    depends_on:
      - postgres

Dockerfile

FROM ruby:2.7.1

WORKDIR /var/workdir
COPY . /var/workdir

# Install NodeJS and Yarn.
RUN curl https://deb.nodesource.com/setup_12.x | bash
RUN curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | apt-key add -
RUN echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list
RUN apt-get update && apt-get install -y nodejs yarn

RUN yarn install --check-files

RUN bundle install

ENV PORT 3000
EXPOSE $PORT
CMD /var/workdir/entrypoint.sh

entrypoint.sh

#!/bin/bash

rm tmp/pids/server.pid

bin/rails db:create
bin/rails db:migrate

bin/rails server -b 0.0.0.0 -p $PORT

config/database.yml

# PostgreSQL. Versions 9.3 and up are supported.

default: &default
  adapter: postgresql
  encoding: unicode
  pool: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
  host: postgres
  port: 5432
  database: <%= Rails.application.credentials.database[:name] %>
  username: <%= Rails.application.credentials.database[:username] %>
  password: <%= Rails.application.credentials.database[:password] %>

development:
  <<: *default

test:
  <<: *default

production:
  <<: *default

.github/workflows/main.yml

name: App

on:
  push:
    branches:
      - main
  pull_request:
    branches:
      - main
jobs:
  build:
    runs-on: ubuntu-latest

    services:
      postgres:
        image: postgres
        env:
          POSTGRES_DB: ${{ secrets.DATABASE_NAME }}
          POSTGRES_USER: ${{ secrets.DATABASE_USER }}
          POSTGRES_PASSWORD: ${{ secrets.DATABASE_PASSWORD }}
        ports:
          - 5439:5432
        # needed because the postgres container does not provide a healthcheck
        options: --health-cmd pg_isready --health-interval 10s --health-timeout 5s --health-retries 5

    steps:
      - name: Checkout repo
        uses: actions/checkout@v2
      - name: Install PostgreSQL package
        run: |
          sudo apt-get -yqq install libpq-dev
      - name: Install yarn
        run: yarn install --check-files
      - uses: actions/setup-node@v1
        with:
          node-version: '12.13.0'
      - name: Setup Ruby
        uses: actions/setup-ruby@v1
        with:
          ruby-version: 2.7.1
      - name: Ruby gem cache
        uses: actions/cache@v2
        with:
          path: vendor/bundle
          key: ${{ runner.os }}-gems-${{ hashFiles('**/Gemfile.lock') }}
          restore-keys: |
            ${{ runner.os }}-gems-
      - name: Build and create DB
        env:
          RAILS_ENV: test
          RAILS_MASTER_KEY: ${{ secrets.RAILS_MASTER_KEY }}
        run: |
          cd server
          gem install bundler
          bundle config path vendor/bundle
          bundle install --jobs 4 --retry 3
          bin/rails db:setup
  deploy:
    runs-on: ubuntu-latest
    needs: build
    steps:
      - uses: actions/checkout@v2
      - name: Docker login
        run: docker login -u ${{ secrets.DOCKER_USER }} -p ${{ secrets.DOCKER_PASSWORD }}
      - name: Build
        run: docker build -t app ./server
      - name: Tags
        run: |
          docker tag app ${{ secrets.DOCKER_USER }}/app
          docker tag app ${{ secrets.DOCKER_USER }}/app:latest
      - name: Push
        run: |
          docker push ${{ secrets.DOCKER_USER }}/app
          docker push ${{ secrets.DOCKER_USER }}/app:latest

UPDATE: By adding the master.key as a secret to Github Actions, the error in the pipeline reads:

rails aborted!
ActiveSupport::MessageEncryptor::InvalidMessage: Cannot load database configuration:
ActiveSupport::MessageEncryptor::InvalidMessage
(...)
Caused by:
OpenSSL::Cipher::CipherError:

When instead I add the test.key as a secret, the original error returns.

This is the docker-compose.yml I created locally to fetch and run the image from Docker Hub:

version: '3'

services:
  app:
    image: user/app:latest

I replaced all user and repo names for this question.