Master Building Docker WordPress Images and Understanding ENTRYPOINT vs CMD

Docker Practical Preparation

Four steps: log in to OSChina Git, fork the docker‑training repository, connect to the server via SSH, and clone the repository.

Build csphere/wordpress:4.2 Image

<code># cd docker-training/wordpress/
# ls -a
.              license.txt           wp-config-sample.php  wp-login.php
..             readme.html           wp-content            wp-mail.php
Dockerfile     wp-activate.php       wp-cron.php           wp-settings.php
.dockerignore  wp-admin              wp-includes           wp-signup.php
index.php      wp-blog-header.php    wp-links-opml.php     wp-trackback.php
init.sh        wp-comments-post.php  wp-load.php           xmlrpc.php

# cat Dockerfile 
from csphere/php-fpm:5.4

add init.sh /init.sh

entrypoint ["/init.sh", "/usr/bin/supervisord", "-n", "-c", "/etc/supervisord.conf"]</code>

Use a

.dockerignore

file to exclude files that should not be packaged.

Base image is

csphere/php-fpm:5.4

.

The

ONBUILD

instruction copies the code into the web root.

init.sh

configures the WordPress MySQL connection, so the database is ready when the container starts.

Build the image:

<code>docker build -t csphere/wordpress:4.2 .</code>

List local images:

<code>docker images</code>

Find the host IP address:

<code>ifconfig eth0</code>

Run the WordPress container:

<code>docker run -d -p 80:80 --name wordpress -e WORDPRESS_DB_HOST=192.168.1.20 -e WORDPRESS_DB_USER=admin -e WORDPRESS_DB_PASSWORD=csphere2015 csphere/wordpress:4.2</code>

-d

runs the container in the background.

-p 80:80

maps host port 80 to container port 80.

--name wordpress

names the container.

-e WORDPRESS_DB_HOST=192.168.1.20

sets the database host IP.

-e WORDPRESS_DB_USER=admin

sets the database user.

-e WORDPRESS_DB_PASSWORD=csphere2015

sets the database password.

The image

csphere/wordpress:4.2

creates the container.

Access

http://your_ip

, choose a language, and complete the WordPress setup.

ENTRYPOINT vs CMD

ENTRYPOINT configures a container to run as an executable. Preferred exec form:

<code>ENTRYPOINT ["executable", "param1", "param2"]</code>

Shell form:

<code>ENTRYPOINT command param1 param2</code>

CMD provides default parameters to

ENTRYPOINT

or runs a command. Preferred exec form:

<code>CMD ["executable", "param1", "param2"]</code>

It can also supply parameters to

ENTRYPOINT

:

<code>CMD ["param1", "param2"]</code>

Or use the shell form:

<code>CMD command param1 param2</code>

Practical test of CMD

<code>vim Dockerfile
FROM centos:centos7.1.1503

CMD ["/bin/echo", "This is test cmd"]</code>

<code>docker build -t csphere/cmd:0.1 .

docker run -it --rm csphere/cmd:0.1
# Output: This is test cmd</code>

Providing a new command at

docker run

overrides the CMD:

<code>docker run -it csphere/cmd:0.1 /bin/bash
# You get a shell</code>

Practical test of ENTRYPOINT

<code>FROM centos:centos7.1.1503

ENTRYPOINT ["/bin/echo", "This is test entrypoint"]</code>

<code>docker build -t csphere/ent:0.1 .

docker run -it csphere/ent:0.1
# Output: This is test entrypoint</code>

By default, ENTRYPOINT cannot be overridden, but it can be replaced using the

--entrypoint

option:

<code>docker run -it --entrypoint=/bin/bash csphere/ent:0.1
# You get a shell</code>

This demonstrates that ENTRYPOINT is fixed unless explicitly changed at run time, while CMD is more flexible.