The Docker Tutorial

I found this interesting tutorial on Laracasts. It's by Andrew Schmelyun.

Episode 1 - What is Docker

Docker is a software platform that uses containers to build, share and run applications.
You can use Docker Desktop.
Alternatively, you can follow these steps Install Docker Engine on Ubuntu and Install Docker Compose.

Now Docker is installed. How can we use it?
First we open a terminal and enter the following command:

docker run hello-world

The command pull down the "hello-world" container from the docker hub.
To remove the container completely just use the following command:

docker run --rm hello-world

or delete it using the delete icon in the Docker desktop application.

To get version information about the pulled container just use the following command:

docker run --rm ubuntu:latest cat /etc/os-release

Episode 2 - Create a Simple Docker File

First we create a custom Dockerfile:

touch Dockerfile
code Dockerfile

The aim of this file is to provide us a set of instructions for docker to use that builds and prepares our container when we use docker run.
Lets think of something we can do with this container. How about to create a container for a JSON Server.

The first thing that has to be included in every Dockerfile is the FROM command:

FROM node:latest

This is followed by the base image that you want to build your container.

Next we use the WORKDIR command to determine in what directory we are going to be in when we run any following commands:

WORKDIR /home/server

If the directory isnt available it will be created.

To execute instructions like installing a npm package we can use the RUN command:

RUN npm install -g json-server

For the JSON Server we need a file for its data:

touch db.json
code db.json

with the following example data:

{
  "posts": [
    {
      "title": "My first Post",
      "author": "web devapps"
    },
    {
      "title": "My second Post",
      "author": "web devapps"
    }
  ],
  "profile": {
    "first_name": "Web",
    "last_name": "Devapps",
    "email": "ich@webdevapps.de"
  }
}

In order to be able to use the data file in our Docker container we can use the COPY command to copy the file into the container:

COPY db.json /home/server/db.json

The last thing we need to do is tell Docker to actually run json-server. There are two ways to do it:

1. ENTRYPOINT
2. CMD

ENTRYPOINT is the main command ran by the Docker container. It can not be overwritten by commands on the host machine instead commands are added to it.

CMD is for additional commands or options that will run after the ENTRYPOINT command.

For now we use the "ENTRYPOINT" command (array notation):

ENTRYPOINT ["json-server", "db.json"]

There is also the alternative shell form:

ENTRYPOINT /bin/sh -c json-server db.json

But we use the exact form as array notation above.

Now its time to build the container:

docker build .

After the Docker container has been created, we can display the docker images with:

docker image list

REPOSITORY	TAG	IMAGE ID	CREATED	SIZE
<none>	<none>	0c3f3aa2ecc3	About a minute ago	1.13GB

We copy the Image ID and put it to use

docker run --rm 0c3f3aa2ecc3

Now we see the "Hello Screen" from json server. Lets see if we can load the domain:

http://localhost:3000

What a surprise we get an error page. What's the problem? The json server is running on port 3000 but that port is isolated inside of the docker container. Its not exposed to our local machine and so we can't send or receive any data from it. We have to change that.

First let's bring down the docker container thats running. In a new terminal tab run:

docker ps

CONTAINER ID	IMAGE	COMMAND	CREATED	STATUS	PORTS	NAMES
1349d3d6440d	0c3f3aa2ecc3	"json-server db.json"	12 minutes ago	Up 12 minutes		trusting_franklin
This command shows all currently running images. We copy the id of our container and run:

docker stop 1349d3d6440d

Let's open up our Dockerfile again:

FROM node:latest

WORKDIR /home/server

RUN npm install -g json-server

COPY db.json /home/server/db.json

ENTRYPOINT ["json-server", "db.json"]

After our COPY Statement we add another command called EXPOSE. This expects a port number and gives our container the ability to map a port from our host machine into the docker container at the port specifed. By default we saw that json server runs on port 3000:

EXPOSE 3000

By default json server runs on localhost but we need to specify a different ip address. In our ENTRYPOINT lets add the --host flag and the address 0.0.0.0:

ENTRYPOINT ["json-server", "db.json", "--host", "0.0.0.0"]

The ip address 0.0.0.0 is the default docker bridge network ip essentially connecting our host machine to the docker instance. Our dockerfile now looks like this:

FROM node:latest

WORKDIR /home/server

RUN npm install -g json-server

COPY db.json /home/server/db.json

EXPOSE 3000

ENTRYPOINT ["json-server", "db.json", "--host", "0.0.0.0"]

Lets rebuild the container:

docker build . --no-cache

The --no-cache flag disregard any cache files that let have changed. Copy the image id and run:

docker run --rm -p 3000:3000 2f269b32d46d

With the -p flag we can choose which port on our local machine we bind that to. -p 3000:3000 means that the first number is the local system and the second number is the docker container. Lets have a look in our browser what happens if we call http://localhost:3000/. Fine we see the json server landing page as expected and see the data we provided.

How we can use the CMD to specify some optional arguments:

Lets try to use the CMD Command with some optional arguments. Bring down the actual running image container. Open the Dockerfile and add under the COPYcommand another COPY command and remove from the ENTRYPOINT command the second argument db.json:

...

COPY db.json /home/server/db.json

COPY old.json /home/server/old.json

ENTRYPOINT ["json-server", "--host", "0.0.0.0"]

CMD ["db.json"]

In our project directory we copy the db.json as old.json

cp db.json old.json

Lets make a few changes to old.json so that we know there is a difference:

{
  "posts": [
    {
      "title": "My alternative first Post",
      "author": "web devapps two"
    },
    {
      "title": "My alternative second Post",
      "author": "web devapps two"
    }
  ],
  "profile": {
    "first_name": "Web",
    "last_name": "Devapps Two",
    "email": "ich2@webdevapps.de"
  }
}

Lets rebuild our docker container. With the -t json-server flag we can specify an image name:

docker build . --no-cache -t json-server

Now we can run with and see the expected landing page with the posts of db.json:

docker run --rm -p 3000:3000 json-server

If we bring the container down and rerun it with the specifying the old.json file at the end:

docker run --rm -p 3000:3000 json-server old.json

Now we see the landing page with the alternative posts from old.json. Nice :-)