Using a Dockerfile to modify a container

In the previous post I modified a CentOS Docker container image to include the man application as a simple example. But it was not obvious looking at the container what modifications, if any, had been made. I wanted instead to specify the installation of man via a Dockerfile so that the modifications to the base OS would be more clear to another user.

First, I create a new empty directory and create a Dockerfile within the directory:

$ mkdir man-centos
$ cd man-centos
$ vi Dockerfile

In the Dockerfile, I specify that I want to use the publicly-available CentOS 7 base image, and then install the man package via yum:

# Start with CentOS 7
FROM centos:7

# Install the man yum module
RUN yum -y install man

Then build and run the container:

$ docker build -t man-centos .
$ docker run -i -t man-centos
[root@6f597c97d72d /]# man man
No manual entry for man

It works!

Running CentOS in a Docker container

I’m just getting started with Docker. I’ve thought for years that containerization is a great idea, but I haven’t actually done anything with containers yet. Time to get started.

I ran through a couple tutorials on the Docker docs site and created a cloud.docker.com account to get some basic familiarity.

I found the CentOS container repository on Docker Hub: https://hub.docker.com/_/centos/

Let’s try running it!

$ docker pull centos
$ docker run centos

Did it do anything? It looks like it did something. At least, it didn’t give me an error. What did it do? How do I access it?

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

Nothing is actively running. That makes sense, because we’re not telling the containerized OS to do anything — it starts, it doesn’t have anything to do, and so it shuts down immediately. Instead we can tell it to run interactively and with a terminal by specifying a couple options:

-i, --interactive
-t, --tty
(“allocate a pseudo-TTY”, i.e. a terminal)
(see docker run --help for details)

$ docker run -i -t centos
[root@4f0b435cdbd7 /]#

I’m in!

What if I want to modify the container? Right now it is pretty bare-bones. For example, this doesn’t even have man installed:

[root@4f0b435cdbd7 /]# man man
bash: man: command not found

[root@4f0b435cdbd7 /]# yum install man
...
[root@4f0b435cdbd7 /]# man man
No manual entry for man

Quite the improvement! Now we need to save our change:

[root@4f0b435cdbd7 /]# exit

$ docker commit 4f0b435cdbd7 man-centos
$ docker run -i -t man-centos

[root@953c512d6707 /]# man man
No manual entry for man

Progress! Now we have a CentOS container where man is already installed. Exciting.

I can’t (that I know of) inspect the container and know whether or not man is installed without running it. That’s fine for many cases, but next I will attempt to figure out how specify via a Dockerfile that man is installed.