Note

This will be a very brief tutorial on Docker: we’ll take a “nginx” docker image and build a simple web server on Docker container. In that process, we can get a quick taste of how Docker is working.

We’ll learn:

  1. How to get an official image.
  2. How to use the files on host machine from our container.
  3. How to copy the files from our host to the container.
  4. How to make our own image – in this tutorial, we’ll make a Dockerfile with a page for render.

Getting Nginx official docker image

To create an instance of Nginx in a Docker container, we need to search for and pull the Nginx official image from Docker Hub. Use the following command to launch an instance of Nginx running in a container and using the default configuration:

$ docker container run --name my-nginx-1 -P -d nginx
f0cea39a8bc38b38613a3a4abe948d0d1b055eefe86d026a56c783cfe0141610

The command creates a container named “my-nginx-1” based on the Nginx image and runs it in “detached” mode, meaning the container is started and stays running until stopped but does not listen to the command line. We will talk about this later how to interact with the container.

The Nginx image exposes ports 80 and 443 in the container and the -P option tells Docker to map those ports to ports on the Docker host that are randomly selected from the range between 49153 and 65535.

We do this because if we create multiple Nginx containers on the same Docker host, we may induce conflicts on ports 80 and 443. The port mappings are dynamic and are set each time the container is started or restarted.

If we want the port mappings to be static, set them manually with the -p option. The long form of the “Container Id” will be returned.

We can run docker ps to verify that the container was created and is running, and to see the port mappings:

$ docker container ps
CONTAINER ID IMAGE COMMAND                 CREATED       STATUS       PORTS                                        NAMES
f0cea39a8bc3 nginx "nginx -g 'daemon off"  5 minutes ago Up 5 minutes 0.0.0.0:32769->80/tcp   my-nginx-1

We can verify that Nginx is running by making an HTTP request to port 32769 (reported in the output from the preceding command as the port on the Docker host that is mapped to port 80 in the container), the default Nginx welcome page appears:

nginx-default-32769.png
$ curl http://localhost:32769
<!DOCTYPE html>
<html>
<head>
...
</html>

We can stop the container and remove:

$ docker container stop f0cea39a8bc3
f0cea39a8bc3

$ docker container ps
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES

$ docker container rm f0cea39a8bc3

If we want to map the container’s port 80 to a specific port of the host, we can use -p with host_port:docker_port:

$ docker container run --name my-nginx-1 -p8088:80 -d nginx
bd39cc4ed6dbba92e85437b0ab595e92c5ca2e4a87ee3e724a0cf04aa9726044

$ docker container ps
CONTAINER ID  IMAGE  COMMAND                 CREATED         STATUS         PORTS                  NAMES
bd39cc4ed6db  nginx  "nginx -g 'daemon of…"  18 seconds ago  Up 18 seconds  0.0.0.0:8088->80/tcp   my-nginx-1
nginx-8088.png

More work on the Nginx Docker Container

Now that we have a working Nginx Docker container, how do we manage the content and the configuration? What about logging?

It is common to have SSH access to Nginx instances, but Docker containers are generally intended to be for a single purpose (in this case running Nginx) so the Nginx image does not have OpenSSH installed and for normal operations there is no need to get shell access directly to the Nginx container. We will use other methods supported by Docker rather than using SSH.

Keep the Content and Configuration on the Docker Host

When the container is created, we can tell Docker to mount a local directory on the Docker host to a directory in the container.

The Nginx image uses the default Nginx configuration, so the root directory for the container is /usr/share/nginx/html. If the content on the Docker host is in the local directory /tmp/nginx/html, we run the command:

$ docker container run --name my-nginx-2 \
-v /tmp/nginx/html:/usr/share/nginx/html:ro -P -d nginx

$ docker container ps
CONTAINER ID  IMAGE  COMMAND                CREATED       STATUS       PORTS                   NAMES
35a4c74ff073  nginx  "nginx -g 'daemon off" 7 seconds ago Up 2 seconds 0.0.0.0:32770->80/tcp   my-nginx-2

Now any change made to the files in the local directory, /tmp/nginx/html on the Docker host are reflected in the directories /usr/share/nginx/html in the container. The :ro option causes these directors to be read only inside the container.

share-volume-32770.png

Copying files from the Docker host

Rather than using the files that kept in the host machine, another option is to have Docker copy the content and configuration files from a local directory on the Docker host when a container is created.

Once a container is created, the files are maintained by creating a new container when the files change or by modifying the files in the container.

A simple way to copy the files is to create a Dockerfile to generate a new Docker image, based on the Nginx image from Docker Hub. When copying files in the Dockerfile, the path to the local directory is relative to the build context where the Dockerfile is located. For this example, the content is in the MyFiles directory under the same directory as the Dockerfile.

Here is the Dockerfile:

FROM nginx
COPY MyFiles /usr/share/nginx/html

We can then create our own Nginx image by running the following command from the directory where the Dockerfile is located:

$ docker image build -t my-nginx-image-1 .
Sending build context to Docker daemon  4.096kB
Step 1/2 : FROM nginx
 ---> be1f31be9a87
Step 2/2 : COPY MyFiles /usr/share/nginx/html
 ---> 507c7d77c45f
Successfully built 507c7d77c45f
Successfully tagged my-nginx-image-1:latest

Note the period (“.”) at the end of the command. This tells Docker that the build context is the current directory. The build context contains the Dockerfile and the directories to be copied.

Now we can create a container using the image by running the command:

$ docker container run --name my-nginx-3 -P -d my-nginx-image-1
150fe5eddb31acde7dd432bed73f8daba2a7eec3760eb272cb64bfdd2809152d

$ docker container ps
CONTAINER ID  IMAGE             COMMAND                  CREATED        STATUS        PORTS                   NAMES
150fe5eddb31  my-nginx-image-1  "nginx -g 'daemon of…"   7 seconds ago  Up 7 seconds  0.0.0.0:32772->80/tcp   my-nginx-3
copied-file-32772.png

Editing (Sharing) files in Docker container : volumes

Actually, the volume of Docker belongs to an advanced level. So, in this section, we only touch the topic briefly.

As we know, we are not able to get SSH access to the Nginx container, so if we want to edit the container files directly we can use a helper container that has shell access.

In order for the helper container to have access to the files, we must create a new image that has the proper volumes specified for the image.

Assuming we want to copy the files as in the example above, while also specifying volumes for the content and configuration files, we use the following Dockerfile:

FROM nginx
COPY content /usr/share/nginx/html
VOLUME /myVolume-1

We then create the new Nginx image (my-nginx-image-2) by running the following command:

$ docker image build -t my-nginx-image-2 .
Sending build context to Docker daemon  4.096kB
Step 1/3 : FROM nginx
 ---> be1f31be9a87
Step 2/3 : COPY MyFiles /usr/share/nginx/html
 ---> Using cache
 ---> 2bddb692d90d
Step 3/3 : VOLUME /myVolume-1
 ---> Running in 699a41ac23c9
Removing intermediate container 699a41ac23c9
 ---> 1037d1f75691
Successfully built 1037d1f75691
Successfully tagged my-nginx-image-2:latest

Now we create an Nginx container (my-nginx-4) using the image by running the command:

$ docker container run --name my-nginx-4 -P -d my-nginx-image-2
8c84cf08dc08a0b00bd04c9b7047ff65f1ec5c910aa6b096a743b18652e128f9

We then start a helper container with a shell and access the content and configuration directories of the Nginx container we created in the previous example by running the command:

$ docker container run -i -t --volumes-from my-nginx-4 --name my-nginx-4-with-volume debian /bin/bash
root@64052ed5e1b3:/# 

This creates an interactive container named my-nginx-4-with-volume that runs in the foreground with a persistent standard input (-i) and a tty (-t) and mounts all the volumes defined in the container my-nginx-4 as local directories in the my-nginx-4-with-volume container.

After the container is created, it runs the bash shell, which presents a shell prompt for the container that we can use to modify the files as needed.

Now, we can check the volume has been mounted:

root@64052ed5e1b3:/# ls           
bin  boot  dev	etc  home  lib	lib64  media  mnt  myVolume-1  opt  proc  root	run  sbin  srv	sys  tmp  usr  var
root@64052ed5e1b3:/#

To check the volume on local machine we can go to /var/lib/docker/volumes. On Mac system, we need to take an additional step to:

$ screen ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/tty

Or for newer system:

screen ~/Library/Containers/com.docker.docker/Data/vms/0/tty

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