How to build docker image for gitlab container registry?

Hello! I creating application with gitlab package registry(dockerhub but local in gitlab).

I created my docker image for laravel only for testins env and next step deploy my app to servers.

But. when I testing deployment jobs I get error:

Composer could not find a composer.json file in /builds/

Docker image for testing:

FROM php:8-fpm-alpine

WORKDIR /var/www

RUN apk update \
    && apk add \
    $PHPIZE_DEPS \
    git \
    zlib-dev \
    libzip-dev \
    zip && \
    rm -rf /var/lib/apt/lists

RUN pecl install  igbinary \
    && docker-php-ext-enable igbinary  \
    && pecl install msgpack \
    && docker-php-ext-enable msgpack  \
    && docker-php-ext-install bcmath pcntl sockets pdo_mysql zip

COPY --from=composer:latest /usr/bin/composer /usr/bin/composer

Gitlab-ci.yml

image: $CI_REPOSITORY/gitlab-ci

stages:
    - composer
    - build
    - syntax
    - phpunit

cache:
    paths:
        - vendor/
        - .env

composer:
    stage: composer
    script:
        - composer install --prefer-dist --no-ansi --no-interaction --no-progress --no-scripts
    only:
        changes:
            - composer.json

# Prepare project
build:
    stage: build
    script:
        - cp .env.testing .env
        - php artisan key:generate
    only:
        changes:
            - .env.testing

syntax:
    stage: syntax
    script:
        - composer dump-autoload
        - vendor/bin/phpcs --error-severity=1 --warning-severity=8 --extensions=php
    only:
        changes:
            - "**/*.php"

phpunit:
    stage: phpunit
    script:
        - vendor/bin/phpunit --bootstrap vendor/autoload.php --configuration phpunit.xml --testsuite Unit
    only:
        changes:
            - composer.json
            - app/**/*
            - config/**/*
            - tests/**/*

So, I wanted to ask, does the application code really need to be stored in the image, or can it be copied there during after pushing to gitlab?

Do I need make something like this:

COPY . /var/www

Or mb this?

docker run --name gitlab-test-ci -v .:/var/www -dp 80:80 gitlab-test-ci

I mean how can I fix error in my application and how can make correctly settings for gitlab package registry?

Thanks