Step 3: Extending a Docker Image

Objective: Create an SSL version of WordPress

Migrating WordPress to an SSL version requires two steps in the correct sequence. Making the changes in the wrong order could require database modifications to regain access to the site.

4.3.1. Change the WordPress URL to HTTPS

Let’s do this first so that you do not lock yourself out of your site.

  1. Open the admin panel of your WordPress site.

    Did you forget your password? You can reset it using Reset admin password.

  2. Go to Settings -> General Settings

  3. Change the values to read https

    ../../_images/updatewpurls.png ../../_images/updatewpurls_ru.png

At this point, you cannot access your site until you configure an SSL transport layer.

4.3.2. Build the new WordPress Image

Extending an existing image is simple. We don’t have to rebuild the entire project. We could clone the WordPress code from GitHub and manually build the project using the base WordPress Dockerfile for Apache, but we will let someone else maintain that Dockerfile. We only need to base our image on an official WordPress image of our choice. For this lab, we will use the latest Apache version (wordpress:apache).

To enable SSL in an image, we need to:

  1. Base our project from an existing image (wordpress:apache)

  2. Enable the Apache modules required for SSL

  3. Copy self-signed certificates

  4. Build our custom image

We’ll start by creating our project directory and files.

  1. Create the directory for our image and other files.

    cd ~
    mkdir wordpress-docker-image
    cd wordpress-docker-image
    touch Dockerfile
  2. The image needs self-signed SSL certificates to work. We will create them using package make-ssl-cert.

    apt-get -y install ssl-cert
    make-ssl-cert generate-default-snakeoil
    # Verify the new SSL certificates exist.
    ls -l /etc/ssl/certs/ssl-cert-snakeoil.pem /etc/ssl/private/ssl-cert-snakeoil.key

    Then, we need to copy the files to our current directory.

    cp -p /etc/ssl/certs/ssl-cert-snakeoil.pem .
    cp -p /etc/ssl/private/ssl-cert-snakeoil.key .
     1root@vps298933:~/wordpress-docker-image# apt-get -y install ssl-cert
     2Reading package lists... Done
     3Building dependency tree
     4Reading state information... Done
     5Suggested packages:
     6  openssl-blacklist
     7The following NEW packages will be installed:
     8  ssl-cert
     90 upgraded, 1 newly installed, 0 to remove and 4 not upgraded.
    10Need to get 17.0 kB of archives.
    11After this operation, 64.5 kB of additional disk space will be used.
    12Get:1 bionic/main amd64 ssl-cert all 1.0.39 [17.0 kB]
    13Fetched 17.0 kB in 0s (442 kB/s)
    14Preconfiguring packages ...
    15Selecting previously unselected package ssl-cert.
    16(Reading database ... 128224 files and directories currently installed.)
    17Preparing to unpack .../ssl-cert_1.0.39_all.deb ...
    18Unpacking ssl-cert (1.0.39) ...
    19Setting up ssl-cert (1.0.39) ...
    20Processing triggers for man-db (2.8.3-2ubuntu0.1) ...
    21root@vps298933:~/wordpress-docker-image# make-ssl-cert generate-default-snakeoil
    22root@vps298933:~/wordpress-docker-image#  ls -l /etc/ssl/certs/ssl-cert-snakeoil.pem /etc/ssl/private/ssl-cert-snakeoil.key
    23-rw-r--r-- 1 root root     1034 Nov 13 22:21 /etc/ssl/certs/ssl-cert-snakeoil.pem
    24-rw-r----- 1 root ssl-cert 1708 Nov 13 22:21 /etc/ssl/private/ssl-cert-snakeoil.key
    25root@vps298933:~/wordpress-docker-image# cp -p /etc/ssl/certs/ssl-cert-snakeoil.pem .
    26root@vps298933:~/wordpress-docker-image# cp -p /etc/ssl/private/ssl-cert-snakeoil.key .
    28root@vps298933:~/wordpress-docker-image# ls -lh
    29total 8.0K
    30-rw-r--r-- 1 root root        0 Nov 13 22:21 Dockerfile
    31-rw-r----- 1 root ssl-cert 1.7K Nov 13 22:21 ssl-cert-snakeoil.key
    32-rw-r--r-- 1 root root     1.1K Nov 13 22:21 ssl-cert-snakeoil.pem
  3. We can now create our Dockerfile.

     1# The base image
     2FROM wordpress:apache
     4# Enabling the SSL module
     5RUN a2enmod ssl
     7# Copy the SSL certs to the image
     8RUN mkdir /etc/apache2/certs
     9COPY ssl-cert-snakeoil.pem /etc/apache2/certs/ssl-cert.pem
    10COPY ssl-cert-snakeoil.key /etc/apache2/certs/ssl-cert.key
  4. Lastly, build and the new image. You can give any name, but this example uses ssl-wordpress.

    docker build -t ssl-wordpress:latest .
     1root@vps298933:~/wordpress-docker-image# docker build -t ssl-wordpress:latest .
     2Sending build context to Docker daemon  6.656kB
     3Step 1/5 : FROM wordpress:apache
     4apache: Pulling from library/wordpress
     5Digest: sha256:20bffad04c9c3e696b3c6fbc48d769c5948718b57af8c9457d9a0f28b5066b4b
     6Status: Downloaded newer image for wordpress:apache
     7 ---> 6edecd0f5c75
     8Step 2/5 : RUN a2enmod ssl
     9 ---> Running in cfa4595ed571
    10Considering dependency setenvif for ssl:
    11Module setenvif already enabled
    12Considering dependency mime for ssl:
    13Module mime already enabled
    14Considering dependency socache_shmcb for ssl:
    15Enabling module socache_shmcb.
    16Enabling module ssl.
    17See /usr/share/doc/apache2/README.Debian.gz on how to configure SSL and create self-signed certificates.
    18To activate the new configuration, you need to run:
    19  service apache2 restart
    20Removing intermediate container cfa4595ed571
    21 ---> 028793b3215f
    22Step 3/5 : RUN mkdir /etc/apache2/certs
    23 ---> Running in 66bf5da33f60
    24Removing intermediate container 66bf5da33f60
    25 ---> 8123001e5c7a
    26Step 4/5 : COPY ssl-cert-snakeoil.pem /etc/apache2/certs/ssl-cert.pem
    27 ---> 872b8b06ddc4
    28Step 5/5 : COPY ssl-cert-snakeoil.key /etc/apache2/certs/ssl-cert.key
    29 ---> 61532a6d1302
    30Successfully built 61532a6d1302
    31Successfully tagged ssl-wordpress:latest
     1root@vps298933:~/wordpress-docker-image# docker images
     2REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
     3ssl-wordpress       latest              61532a6d1302        23 minutes ago      546MB
     4pandoc              latest              79ad25bc9c62        4 hours ago         1.03GB
     5pandoc              default             7eb0798f4080        10 hours ago        720MB
     6node                latest              f1974cfde44f        40 hours ago        935MB
     7mariadb             latest              2ab9d091310d        44 hours ago        414MB
     8nextcloud           latest              3ed6ea445002        7 days ago          811MB
     9wordpress           apache              6edecd0f5c75        7 days ago          546MB
    10wordpress           latest              6edecd0f5c75        7 days ago          546MB
    11redis               latest              62f1d3402b78        2 weeks ago         104MB
    12node                6.10                3f3928767182        3 years ago         661MB

4.4.3. Add SSL Configuration

The Apache webserver running in the new image called ssl-wordpress supports both HTTP and HTTPS. We need to add a configuration file for the WordPress site to use SSL. We do the same thing when we create an Nginx site.

  1. First, we need to create an Apache SSL configuration for WordPress to listen on port 433 (SSL). We will store these files in a volume directory called apache.

    # Create the volume directory and file
    cd ~/wordpress-docker
    mkdir apache
    touch apache/site.conf

    Add this information to apache/site.conf.

     1<VirtualHost *:80>
     2    #ServerName
     3    DocumentRoot /var/www/html
     5    ErrorLog ${APACHE_LOG_DIR}/error.log
     6    CustomLog ${APACHE_LOG_DIR}/access.log combined
     8<IfModule mod_ssl.c>
     9    <VirtualHost _default_:443>
    10       #ServerName
    11        DocumentRoot /var/www/html
    13        ErrorLog ${APACHE_LOG_DIR}/error.log
    14        CustomLog ${APACHE_LOG_DIR}/access.log combined
    16        SSLEngine on
    17        SSLCertificateFile      /etc/apache2/certs/ssl-cert.pem
    18        SSLCertificateKeyFile   /etc/apache2/certs/ssl-cert.key
    20        <FilesMatch "\.(cgi|shtml|phtml|php)$">
    21            SSLOptions +StdEnvVars
    22        </FilesMatch>
    23        <Directory /usr/lib/cgi-bin>
    24            SSLOptions +StdEnvVars
    25        </Directory>
    26    </VirtualHost>
  2. Next, we need to modify the docker-compose.yml so that the running container can use SSL.

    1. Change the image to ssl-wordpress:latest

    2. Create a volume for our custom site.conf file.

    3. Tell the container to listen on port 443 by adding a new port map (20843:443).


      We can the numbers 8 and 4 to differentiate between HTTP and HTTPS maps.

      • HTTP sites use port 80 or 8080. We use 208xx

      • HTTPS sites use port 443 or 4443. We use 204xx

    . . .
        - db
        - redis
      image: ssl-wordpress:latest
        - ./wordpress:/var/www/html
        - ./apache/site.conf:/etc/apache2/sites-enabled/000-default.conf
        - "20851:80"
        - "20435:443"
      restart: always
    . . .
  3. Restart and verify that the container responds to HTTPS requests.

    docker-compose down && docker-compose up -d
    curl -k --head https://localhost:20435
    root@vps298933:~/wordpress-docker# docker-compose down && docker-compose up -d
    Stopping wordpressdocker_wordpress_1 ... done
    Stopping wordpressdocker_redis_1     ... done
    Stopping wordpressdocker_db_1        ... done
    Removing wordpressdocker_wordpress_1 ... done
    Removing wordpressdocker_redis_1     ... done
    Removing wordpressdocker_db_1        ... done
    Removing network wordpressdocker_default
    Creating network "wordpressdocker_default" with the default driver
    Creating wordpressdocker_redis_1 ...
    Creating wordpressdocker_db_1 ...
    Creating wordpressdocker_redis_1
    Creating wordpressdocker_redis_1 ... done
    Creating wordpressdocker_wordpress_1 ...
    Creating wordpressdocker_wordpress_1 ... done
    root@vps298933:~/wordpress-docker# curl -k --head https://localhost:20435
    HTTP/1.1 301 Moved Permanently
    Date: Fri, 13 Nov 2020 17:41:38 GMT
    Server: Apache/2.4.38 (Debian)
    X-Powered-By: PHP/7.4.12
    X-Redirect-By: WordPress
    Location: https://localhost/
    Content-Type: text/html; charset=UTF-8
  4. The last step is to configure Nginx to operate on SSL. Change proxy_pass to HTTPS.

    Edit your blog Nginx site: nano /etc/nginx/sites-available/

     1server {
     2    listen 80;
     4    server_name;
     6    location / {
     7        #proxy_pass http://localhost:20851;
     8        proxy_pass https://localhost:20435;
     9        proxy_set_header Host $host;
    10        proxy_set_header X-Real-IP $remote_addr;
    11        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
    12    }

    Restart Nginx and obtain an SSL certificate.

    nginx -t
    systemctl restart nginx
    certbot --nginx

View the page in your browser to verify that WordPress has SSL.