It’s a great tool to have in the toolbox. Might take some time to wrap your head around, but coming from vm’s you already have most of the base understanding.
From a VM user’s perspective, some translations:
Dockerfile = script to set up a VM from a base distro, and create a checkpoint that is used as a base image for starting up vm’s
A container is roughly similar to a running VM. It runs inside the host os, jailed, which account for it’s low overhead.
When a container is killed, every file system change gets thrown out. Certain paths and files can be mapped to host folders / storage to keep data between restarts.
Containers run on their own internal network. You can specify ports to nat in from host interface to containers.
Most service setup is done by specifying environment variables for the container, or mapping in a config file or folder.
Since the base image is static, and config is per container, one image can be used to run multiple containers. So if you have a postgres image, you can run many containers on that image. And specify different config for each instance.
Docker compose is used for multiple containers, and their relationship. For example a web service with a DB, static file server, and redis cache. Docker compose also handles things like setting up a unique network for the containers, storage volumes, logs, internal name resolution, unique names for the containers and so on.
A small tip: you can “exec” into a running container, which will run a command inside that container. Combined with interactive (-i) and terminal (-t) flags, it’s a good way to get a shell into a running container and have a look around or poke things. Sort of like getting a shell on a VM.
One thing that’s often confusing for new people are image tags. Partially because it can mean two things. For example “postgres” is a tag. That is attached to an image. The actual “name” of an image is it’s Sha sum. An image can have multiple tags attached. So far so good, right?
Now, let’s get complicated. The actual tag, the full tag for “postgres” is actually “docker.io/postgres:latest”. You see, every tag is a URL, and if it doesn’t have a domain name, docker uses it’s own. And then we get to the “: latest” part. Which is called a tag. Yup. All tags have a tag. If one isn’t given, it’s automatically set to “latest”. This is used for versioning and different builds.
For example postgres have tags like “16.1” which points to latest 16.1.x version image, built on postgres maintainers’ preferred distro. “16.1-alpine” that point to latest Alpine based 16.1.x version. “16” for latest 16.x.x version, “alpine” for latest alpine based version, be it 16 or 17 or 18… and so on. You can find more details here.
The images on docker hub are made by … well, other people. Often the developers of that software themselves, sometimes by docker, sometimes by random people. You can make your own account there, it’s free. If you do, make an image and pushes it, it will be available as shdwdrgn/name - if it doesn’t have a user component it’s maintained / sanctioned by docker.
You can also run your own image repository service, as long as it has https with valid cert. Then it will be yourdomain.tld/something
So that was a brief introduction to the strange World of docker. Docker is a for profit company, btw. But the image format is standardized, and there’s fully open source ways to make and run images too. At the top of my head, podman and Kubernetes.