Docker Cookbook

2.3 Writing Your First Dockerfile | 41

---

CMD
as an argument to 
docker run
. Let’s build a new image by using the 
CMD
instruc‐
tion, like so:
FROM ubuntu:14.04
CMD ["/bin/echo" , "Hi Docker !"]
Let’s build it and run it:
$ docker build .
...
$ docker run eff764828551
Hi Docker !
In the preceding build command, you specified the root directory.
The Dockerfile that you just created was automatically used for the
build. If you want to do a build of an image based on a Dockerfile
that is in a different location, you use the 
-f
option of 
docker
build
and specify the path.
It looks the same, but if you pass a new executable as an argument to the 
docker run
command, this command will be executed instead of the 
/bin/echo
defined in the
Dockerfile:
$ docker run eff764828551 /bin/date
Thu Dec 11 02:49:06 UTC 2014
Discussion
A Dockerfile is a text file that represents the way a Docker image is built and what
happens when a container is started with this image. Starting with three simple
instructions, you can build a fully functioning container: 
FROM
, 
ENTRYPOINT
, 
CMD
. Of
course, this is quite limited in this recipe. Read the Dockerfile 
reference
 to learn
about all the other instructions, or go to 
Recipe 2.4
for a more detailed example.
The CentOS project maintains a large set of Dockerfile examples.
Check out this 
repository
and run a few of their examples to get
more familiar with Dockerfile files.
Remember that 
CMD
can be overwritten by an argument to 
docker run
, while 
ENTRY
POINT
can be overwritten only by using the 
--entrypoint
option of 
docker run
.
Also, you saw that after a command is finished, the container exits. A process that
you want to run in a container needs to run in the foreground; otherwise, the con‐
tainer will stop.