Using docker to build software testing environments

I’ve been on the docker train for quite some time. While the benefits of running production workloads in containers is well known, I find docker just as valuable for evaluating and testing new software on my laptop. I’ll use this blog post to walk through how I build transient test environments for software evaluation.

Docker is based around images (Fedora, CentOS, Ubuntu, etc.), and these images can be created and customized through the use of a Dockerfile. The Dockerfile contains statements to control the OS that is used, the software that is installed and post configuration. Here is a Dockerfile I like to use for building test environments:

$ cat Dockerfile

FROM centos:7
MAINTAINER Matty

RUN yum -y update
RUN yum -y install openssh-server openldap-servers openldap-clients openldap
RUN sed -i 's/PermitRootLogin without-password/PermitRootLogin yes/' /etc/ssh/sshd_config
RUN echo 'root:XXXXXXXX' | chpasswd

RUN /usr/bin/ssh-keygen -t rsa -f /etc/ssh/ssh_host_rsa_key -C '' -N ''
RUN /usr/bin/ssh-keygen -t rsa -f /etc/ssh/ssh_host_dsa_key -C '' -N ''

EXPOSE 22
CMD ["/usr/sbin/sshd", "-D"]

To create an image from this Dockerfile you can use docker build:

$ docker build -t centos:7 .

The “-t” option assigns a tag to the image which can be referenced when a new container is instantiated. To view the new image you can run docker images:

$ docker images centos
REPOSITORY          TAG                 IMAGE ID            CREATED             SIZE
centos              7                   4f798f95cfe1        8 minutes ago       414.8 MB
docker.io/centos    6                   f07f6ca555a5        3 weeks ago         194.6 MB
docker.io/centos    7                   980e0e4c79ec        3 weeks ago         196.7 MB
docker.io/centos    latest              980e0e4c79ec        3 weeks ago         196.7 MB

Not to have some fun! To create a new container we can use docker run:

$ docker run -d -P -h foo --name foo --publish 2222:22 centos:7
f84477722896b2701506ee65a3f5a909199675a9cd591f3591e906a8795eba5c

This instantiates a new CentOS container with the name (–name) foo, the hostname (-h) foo and uses the centos:7 image I created earlier. It also maps (–publish) port 22 in the container to port 2222 on my local PC. To access the container you can fire up SSH and connect to port 2222 as root (this is a test container so /dev/null the hate mail):

$ ssh root@localhost -p 2222
root@localhost's password: 
[root@foo ~]# 

Now I can install software, configure it, break it and debug issues all in an isolated environment. Once I’m satisfied with my testing I can stop the container and delete it:

$ docker stop foo
foo

$ docker rm foo

I find that running an SSH daemon in my test containers is super valuable. For production I would take Jérôme’s advice and look into other methods for getting into your containers.

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