Docker MiniCrashCourse

Kenneth Fossen

4 min readFeb 17, 2021

Mini crash course for Docker and a simple C# application.

The whole guide and code can be found here to follow

Goal is to build a docker container and use docker-compose to utilize the docker container in the project.

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Install Docker on your machine
Clone this repo and follow the guide

We are using the WeatherApi from Microsoft to demonstrate how to build a container that will expose its services on a port 8080.

This project is using C#, but we don’t need any tools on our machine to build this, since we are going to use the docker to do this for us.

docker-compose up -d docker-compose down docker-compose build

Test that the container is working

A Dockerfile is a recipe for how to build and/or run your application in a container.

For our building image we are to be using this image: mcr.microsoft.com/dotnet/sdk:latest For our running image we are to be using this image: mcr.microsoft.com/dotnet/aspnet:latest

More information about these images can be found here: mcr.microsoft.com/dotnet/sdk mcr.microsoft.com/dotnet/aspnet

We are going to use a Dockerfile and use use these aliases FROM <image>:<tag> as builder and then use FROM <image>:<tag> as runner to create a two step build and run. They can be named anything, this is just to illustrate.

Note: If your application is not compiled, and don’t need a build step, e.g. Python Flask webpage, React app, then it would suffice with a runner only image.

Build steps are done to reduce the size of the end image. E.g. if the application is written in Rust, keeping the build and the running image the same. The size of the container will be huge. Meaning, every time there is a change, you will have to download all this locally to get it down. Rust build image may, depending on your build, be 1Gb+ if not more.

With that said, lets build the container. In the root of the WeatherApi project, where the Dockerfile is placed we are to run these commands.

Example command

docker build . --tag weather-api:test

Result of this command should look something like this:

...snip...Successfully built 12042a3b5853
Successfully tagged weather-api:latest

Since it was reported successfully build we can use this container.

Lets just run it. When we do this, we wanna just give it a name for convenience and expose the ports it is supposed to use.

$ docker run -p 8080:80 --name weatherapi weather-api:latest info: Microsoft.Hosting.Lifetime[0] Now listening on: http://[::]:80 info: Microsoft.Hosting.Lifetime[0] Application started. Press Ctrl+C to shut down. info: Microsoft.Hosting.Lifetime[0] Hosting environment: Development info: Microsoft.Hosting.Lifetime[0] Content root path: /app

Open your favourite browser and go to this URL http://localhost:8080/swagger/

Docker-compose is to orchestrate e.g. the launch of a project. Lets say that this API is relying on a backend database and this has to start before we can access the API.

We are just going to simulate that these two components are connected and talking together over code.

Let’s first create the docker-compose.yml for our api and try to launch this first.

The docker-compose.yml is living at the root, outside WeatherApi folder. It can be placed otherplaces, but we need to be in the same folder as it is to run the docker-compose command.

Check that you are in the same dir as the docker-compose.yml and write the following.

$ docker-compose up 
... snip 
Creating docker-crashcourse_weather-api_1 ... done 
Attaching to docker-crashcourse_weather-api_1 
weather-api_1 | info: Microsoft.Hosting.Lifetime[0] 
weather-api_1 | Now listening on: http://[::]:80 
weather-api_1 | info: Microsoft.Hosting.Lifetime[0] 
weather-api_1 | Application started. Press Ctrl+C to shut down. weather-api_1 | info: Microsoft.Hosting.Lifetime[0] weather-api_1 | Hosting environment: Development 
weather-api_1 | info: Microsoft.Hosting.Lifetime[0] weather-api_1 | Content root path: /app

This command will if the image does not exist, build it and then launch it. Since we didn’t use the -d switch, we are stuck in this terminal until we press `CTRL-C`` When we do this the service shuts down.

When using the -d, it may look like this:

$ docker-compose up -d 
Starting docker-crashcourse_weather-api_1 ... done 
$ docker-compose down 
Stopping docker-crashcourse_weather-api_1 ... done 
Removing docker-crashcourse_weather-api_1 ... done 
Removing network docker-crashcourse_default

Using docker-compose for a single container is very usefull. But since we are to add a database and claim that the WeatherApi is depending on this database service.

So we are going to insert the following into the docker-compose.yml.

Now, we are going to run the docker-compose command again and see that the database is also starting.

$ docker-compose up ... lost of text weather-db_1 | Version: '10.4.12-MariaDB-1:10.4.12+maria~bionic' socket: '/var/run/mysqld/mysqld.sock' port: 3306

So this is all good and everything. But now the database and database startet at the same time and that is not good.

But we can add depends_on: for the weather-api service in the docker-compose.yml file.

And when you then run docker-compose you should see that the services are starting in order.

$ docker-compose up 
... snip .. 
weather-db_1 | 2021-02-17 19:02:26 0 [Note] mysqld: ready for connections. weather-db_1 | Version: '10.4.12-MariaDB-1:10.4.12+maria~bionic' socket: '/var/run/mysqld/mysqld.sock' port: 3306 mariadb.org binary distribution weather-api_1 | info: Microsoft.Hosting.Lifetime[0] weather-api_1 | Now listening on: http://[::]:80 ... snip...

Originally published at https://github.com.

Docker MiniCrashCourse

Example command

Written by Kenneth Fossen