Composer install doesn't install packages when running in Dockerfile - docker

Question

I cannot seem to run composer install in Dockerfile but I can in the container after building an image and running the container.
Here's the command from Dockerfile:
RUN composer require drupal/video_embed_field:1.5
RUN composer install --no-autoloader --no-scripts --no-progress
The output is:
Loading composer repositories with package information
Installing dependencies (including require-dev) from lock file
Nothing to install or update
But after running the container with docker-compose:
...
drupal:
image: docker_image
container_name: container
ports:
- 8081:80
volumes:
- ./container/modules:/var/www/html/web/modules
links:
# Link the DB container:
- db
running docker exec composer install will install the packages correctly:
Loading composer repositories with package information
Installing dependencies (including require-dev) from lock file
Package operations: 1 installs, 0 updates, 0 removals
...
Generating autoload files
I am assuming that the composer.json and composer.lock files are correct because I can run the composer install command in the container without any further effort, but only after the container is running.
Update
Tried combining the composer commands with:
RUN composer require drupal/video_embed_field:1.5 && composer install
Same issue, "Nothing to install or update". Ultimately I would like to continue using seperate RUN statements in Dockerfile to take advantage of docker caching.

Answer 1

Your issue is coming from the fact that, docker-compose is meant to orchestrate multiple docker container build and run at the same time and it somehow is not really showing easily what it does behind the scene to people starting with docker.
Behind a docker-compose up there are four steps:
docker-compose build if needed, and if there is no existing image(s) yet, create the image(s)
docker-compose create if needed, and if there is no container(s) existing yet, create the container(s)
docker-compose start start existing container(s)
docker-compose logs logs stderr and stdout of the containers
So what you have to spot on there, is the fact that action contained into you Dockerfile are executed at the image creation step.
When mounting folders is executed at start of containers step.
So when you try to use a RUN command, part of the image creation step, on files like composer.lock and composer.json that are mounted on starting step, you end up having nothing to install via composer because your files are not mounted anywhere yet.
If you do a COPY of those files that may actual get you somewhere, because you will then have the composer files as part of your image.
This said, be careful that the mounted source folder will totally override the mounting point so you could end up expecting a vendor folder and not have it.
What you should ideally do is to have it as the ENTRYPOINT, this one happens at the last step of the container booting.
Here is for a little developing comparison: a docker image is to a docker container what a class is to an instance of an class — an object.
Your container are all created from images built possibly long time before.
Most of the steps in your Dockerfile happens at image creation and not at container boot time.
While most of the instruction of docker-compose are aimed at the automatisation of the container build, which include the mounting of folders.

Answer 2

Just noting a docker-compose.yml approach to the issue when the volume mount overwrites the composer files inside the container:
docker-compose.yml
environment:
PROJECT_DIR: /var/www/html
volumes:
- ./php/docker/entrypoint/90-composer-install.sh:/docker-entrypoint-init.d/90-composer-install.sh
composer-install.sh
#!/usr/bin/env bash
cd ${PROJECT_DIR}
composer install
This runs composer install after the build, using the docker-entrypoint-init.d shell script