posts docker 2019-10-05

Why Docker after all?

You see, the very first time I've heard about it, i thought it was... Silly.

I know how to configure a database. Kinda like it.

i know how to install java, node, all the fun things in the party.

Then last week i had no choice but actually use these things and, well...

It's not that bad.

In fact, the following line sets postgresql up in a single stroke, instead the long path of steps that i know by heart.

docker run -p 5432:5432 -it sombriks/postgres_pt_br

Also, this is not about me. It's about the others.

Many of us already faced how hard is to explain to others how to set up some environment in order to get people working.

Write it down isn't enough, there are plenty of blogs over the internet, yet people get stuck with something.

Docker is nice because put people to work became easier.

Basic concepts

On this land you have two major concepts: containers and images.

Images are machine definitions. You likely will describe them with a Dockerfile and a context folder. Keep in mind that the folder where you're storing the file is important.

Usually you have a folder with the Dockerfile inside it.

To build the image simply use the docker build command:

cd my-new-machine
docker build .

Containers are the "running machine". you can either run them on the foreground using the -it arguments or put them on the background with the -d argument.

You only can run containers from images you know the name or you have built. In both cases, you must have access to the docker registry.

You can also attach to a running container by doing something like this:

docker exec -it placid_torvalds /bin/bash

Where placid_torvalds is the name of the running container and /bin/bash is the shell executable inside the container.

Funny fact is the names given by the docker to containers, :-)

The Dockerfile

This file is a recipe.

It can be a bit confusing sometimes, like the difference between RUN, CMD and ENTRYPOINT, but usually it is pretty straightforward.

It defines the image.

Images can inherit from other images using the FROM statement.

Port binding and mount volumes

When issuing a docker run command you awlays can add -p and -v parameters:

docker run -it -p 8080:8080 -v /path/on/host:/path/on/container:z drunk_swan

The :z thing is to solve SELinux issues.

The -p 8080:8080 is pretty much the -v, first port is on host, second is on container.

Volumes are nice if you want to share content with the container. Files from some project you're developing, for example.

Publishing your images

Do the docker login so you can create, tag and commit your images to the public registry.

No need to mention to avoid to publish sensitive information on public repositories :-)

That's it, things can be simplified once you decide to accept docker on your life.