That Dockerfile mentions both the
composer.json file used to specify which dependencies to install, as well as the
composer.lock file, which is used to make sure to install the same set of dependencies on different systems (same versions). You can get more information on the lock file at https://getcomposer.org/doc/01-basic-usage.md#composer-lock-the-lock-file
However, if that file is not yet available (because you didn't install dependencies locally yet), you'd need to create it or modify the Dockerfile.
To create the lockfile modify the Dockerfile and replace
COPY composer.json composer.lock ./ with only
COPY composer.json ./. The build the Docker image and create the lockfile by running the following commands
docker build --tag my-php-application .
docker run -it --rm -v $(pwd):/app my-php-application composer install
The second command mounts your current directory to the
/app directory in the Docker container and then runs
composer install to install the dependencies. Because
composer install will write the lockfile and you mounted the directory to the Docker container you will end up with the
composer.lock file in your local directory.
Once that's done you can revert the Dockerfile to the version including the lockfile and are ready to go.