tundra authored on 9 Aug
ansible add python3-pip to ubuntu dockerfile 3 months ago
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Docker Sandboxes The Easy Way

Docker has become wildly successful for implementing all manner of fast-boot/fast-destroy emphemeral computing. Normally, you configure a container to do one important thing - run a web server, host a Jenkins instance ... Then just fire-and-forget afterwards.

But you can also use docker to build general purpose sandboxes. To do this, you need to make it easy to log into a running container just like a "real" VM or server. This repo provides a fast track to doing just that.

Things You Can Learn From This

  • How to build a docker image from a dockerfile
  • How to tag docker images
  • How to use ansible to create and destroy a docker network
  • How to use ansible to create, restart, and destroy docker servers
  • How host and ssh keys are managed on a docker instance
  • How to enable ssh access to a docker instance
  • How to share files between docker containers or between the host and a container

But this is not a "toy" system. What you see here is a public subset of what we use all the time here at the TundraWare Intergalactic HQ. We use this for software development, testing new distributed computing ideas, and doing custom builds in a sanitized environment.

Prep Work: What You Need To Do First

The content of this repo assumes you have done several things:

  • You've got docker already running on your machine
  • You've made docker access available to your own login
  • You've got ansible installed on your machine
  • /shared exists on your host machine with permissions 1777

Quickstart For The Impatient

Here's the 10,000 foot view of what you'll have to do once the Prep Work above is done:

  • Configuring sandbox hostname resolution
  • Build a docker image from a dockerfile
  • Use ansible to start a docker network and the sandboxes
  • Login to your running sandboxes

Configuring Sandbox Hostname Resolution

Various parts of this repo assume that there are (up to) 10 running sandboxes whose names are dockersand1 through dockersand10. For this to work, you have to configure name resolution to properly associate these names with their equivant IP addresses.

Most likely, you don't have control of your DNS configuration. The easy way around this is to add the entries you find in dockerfiles/common/etc/dockersand.hosts to your own /etc/hosts file.

Building The docker Images

Getting a docker container running requires it to be built from an "image". Images are built from something called a "dockerfile". It is this file that specifies which Linux distro on which to base the image. The file also specifies any special configuration or software installation you want in your containers. By setting up the image with this stuff in it ahead of time, it will be present every time you start a new container.

There are two dockerfiles in this repo. To build the corresponding images, do this:

cd dockerfiles
./build-img.sh dockersand-centos7
./build-img.sh dockersand-ubuntu

Start The docker Network And Sandboxes

The creation and destruction of the sandboxes is automated using ansible "playbooks".

In each case you are creating/destroying 10 separate sandboxes.

To build the sandboxes and their network:

cd ansible
ansible-playbook -i inventory/dockersand playbooks/dockersand/dockersand_build.yml

To destroy the sandboxes and their network:

cd ansible
ansible-playbook -i inventory/dockersand playbooks/dockersand/dockersand_destroy.yml

To rebuild the sandboxes and their network:

cd ansible
ansible-playbook -i inventory/dockersand playbooks/dockersand/dockersand_rebuild.yml

By default, both the build and rebuild create sandboxes based on the centos7 image. But you can override this on the command line to specify a different image. Just add this to the end of the playbook command line:

--extra-vars "dockersand_image=dockersand-ubuntu"

Logging In

These sandboxes are setup so you can login from your host machine into the running sandboxes using ssh keys. You will find the keys under dockerfiles/common/.ssh/. There is also an ssh configuration stanza you should add to your own ~/.ssh/config to get your client to use the proper key.

However, it is also possible to login using name (test) and password (test).

In general, once you've properly set up your own .ssh/config and installed keys, you'll do something like this:

ssh dockersand8

Once you are logged in, you can promote yourself to root using the sudo command without any further password required.

Sharing Files

The sandboxes are created to share the /shared directory with the host machine. Any file you put there is visible from any of the sandboxes and/or the host machine. This makes it easy to share or move data between the host and any of the sandboxes or between the sandboxes themselves.


Not only is this tooling useful for building and using sandboxes, it's a good way to learn how docker and ansible work. Take the time to explore the various ansible files and dockerfile specifications. There are comments throughout to help explain what's going on and why.

Here are a few ideas of how to expand on what you see here:

  • Try creating your own, new dockerfile for a different distro like, say, debian or arch.

  • Find where the docker network subnet is specified and change it to something else. Don't forget to update /etc/hosts accordingly.

  • While in one sandbox, ssh into another. Notice that this just works. That's because the images are built with the proper ssh keys in place everywhere - user and host. Thus, every container has them. Notice that the name-to-IP association does not exist in the container's own /etc/hosts. Do some research to figure out why it isn't needed.

  • The dockerfiles currently load a lot of software by default. Try factoring this out into separate ansible playbooks to load the software after the sandboxes are up and running. You'll have to parameterize it to account for the different software installation models and package names in the different distros.

  • You'll notice that there is no dockersand0. You can reasonably guess that if such an endpoint existed, its IP would be one digit lower than the IP for dockersand1. Try logging into that IP and see what is there. You'll be surprised ..