I'm having difficulties understanding docker. No matter how many tutorials I watch, guides I read, for me docker-compose is like being able to define multiple Dockerfiles, ie multiple containers. I can define environment variables in both, ports, commands, base images.

I read in other questions/discussions that Dockerfile defines how to build an image, and docker-compose is how to run an image, but I don't understand that. I can build docker containers without having to have a Dockerfile.

It's mainly for local development though. Does Dockerfile have an important role when deploying to AWS for example (where it's probably coming out of the box for example for EC2)?

So the reason why I can work locally with docker-compose only is because the base image is my computer (sorting out the task Dockerfile is supposed to do)?

CodePudding user response：

Think about how you'd run some program, without Docker involved. Usually it's two steps:

1. Install it using a package manager like apt-get or brew, or build it from source
2. Run it, without needing any of its source code locally

In plain Docker without Compose, similarly, you have the same two steps:

1. docker pull a prebuilt image with the software, or docker build it from source
2. docker run it, without needing any of its source code locally

I'd aim to have a Dockerfile that creates an immutable copy of your image, with all of its source code and library dependencies as part of the image. The ideal is that you can docker run your image without -v options to inject source code or providing the command at the docker run command line.

The reality is that there are a lot of moving parts: you probably need to docker network create a network to get containers to communicate with each other, and use docker run -e environment variables to specify host names and database credentials, and launch multiple containers together, and so on. And that's where Compose comes in: instead of running a series of very long docker commands, you can put all of the details you need in a docker-compose.yml file, check it in, and run docker-compose up to get all of those parts put together.

So, do:

• Use Compose to start multiple containers together
• Use Compose to write down complex runtime options like port mappings or environment variables with host names and credentials
• Use Compose to build your image and start a container from it with a single command
• Build your application code and a standard CMD to run it into your Dockerfile.