Docker is a really nifty tool. It vastly lowers the barrier to distributing and executing applications. It forces people to think about building server side code as a collection of discrete applications and services. When it was released, I instantly realized its potential, including for uses it wasn't primary intended for, such as applications in automated build and test environments.

Over the months, Docker's feature set has grown and many of its shortcomings have been addressed. It's more usable than ever. Most of my early complaints and concerns have been addressed or are actively being addressed.

But one supposedly solved part of Docker still bothers me: image creation.

One of the properties that gets people excited about Docker is the ability to ship execution environments around as data. Simply produce an image once, transfer it to a central server, pull it down from anywhere, and execute. That's pretty damn elegant. I dare say Docker has solved the image distribution problem. (Ignore for a minute that the implementation detail of how images map to filesystems still has a few quirks to work out. But they'll solve that.)

The ease at which Docker manages images is brilliant. I, like many, was overcome with joy and marvelled at how amazing it was. But as I started producing more and more images, my initial excitement turned to frustration.

The thing that bothers me most about images is that the de facto and recommended method for producing images is neither deterministic nor results in minimal images. I strongly believe that the current recommended and applied approach is far from optimal and has too many drawbacks. Let me explain.

If you look at the Dockerfiles from the official Docker library (examples: Node, MySQL), you notice something in common: they tend to use apt-get update as one of their first steps. For those not familiar with Apt, that command will synchronize the package repository indexes with a remote server. In other words, depending on when you run the command, different versions of packages will be pulled down and the result of image creation will differ. The same thing happens when you clone a Git repository. Depending on when you run the command - when you create the image - you may get different output. If you create an image from scratch today, it could have a different version of say Python than it did the day before. This can be a big deal, especially if you are trying to use Docker to accurately reproduce environments.

This non-determinism of building Docker images really bothers me. It seems to run counter to Docker's goal of facilitating reliable environments for running applications. Sure, one person can produce an image once, upload it to a Docker Registry server, and have others pull it. But there are applications where independent production of the same base image is important.

One area is the security arena. There are many people who are justifiably paranoid about running binaries produced by others and pre-built Docker images set off all kinds of alarms. So, these people would rather build an image from source, from a Dockerfile, than pull binaries. Except then they build the image from a Dockerfile and the application doesn't run because of an incompatibility with a new version of some random package whose version wasn't pinned. Of course, you probably lost numerous hours tracing down this obscure reason. How frustrating! Determinism and verifiability as part of Docker image creation help solve this problem.

Deterministic image building is also important for disaster recovery. What happens if your Docker Registry and all hosts with copies of its images go down? If you go to build the images from scratch again, what guarantee do you have that things will behave the same? Without determinism, you are taking a risk that things will be different and your images won't work as intended. That's scary. (Yes, Docker is no different here from existing tools that attempt to solve this problem.)

What if your open source product relies on a proprietary component that can't be legally distributed? So much for Docker image distribution. The best you can do is provide a base image and instructions for completing the process. But if that doesn't work deterministically, your users now have varying Docker images, again undermining Docker's goal of increasing consistency.

My other main concern about Docker images is that they tend to be large, both in size and in scope. Many Docker images use a full Linux install as their base. A lot of people start with a base e.g. Ubuntu or Debian install, apt-get install the required packages, do some extra configuration, and call it a day. Simple and straightforward, yes. But this practice makes me more than a bit uneasy.

One of the themes surrounding Docker is minimalism. Containers are lighter than VMs; just ship your containers around; deploy dozens or hundreds of containers simultaneously; compose your applications of many, smaller containers instead of larger, monolithic ones. I get it and am totally on board. So why are Docker images built on top of the bloaty excess of a full operating system (modulo the kernel)? Do I really need a package manager in my Docker image? Do I need a compiler or header files so I can e.g. build binary Python extensions? No, I don't, thank you.

As a security-minded person, I want my Docker images to consist of only the files they need, especially binary files. By leaving out non-critical elements from your image and your run-time environment, you are reducing the surface area to attack. If your application doesn't need a shell, don't include a shell and don't leave yourself potentially vulnerable to shellshock. I want the attacker who inevitably breaks out of my application into the outer container to get nothing, not something that looks like an operating system and has access to tools like curl and wget that could potentially be used to craft a more advanced attack (which might even be able to exploit a kernel vulnerability to break out of the container). Of course, you can and should pursue additional security protections in addition to attack surface reduction to secure your execution environment. Defense in depth. But that doesn't give Docker images a free pass on being bloated.

Another reason I want smaller containers is... because they are smaller. People tend to have relatively slow upload bandwidth. Pushing Docker images that can be hundreds of megabytes clogs my tubes. However, I'll gladly push 10, 20, or even 50 megabytes of only the necessary data. When you factor in that Docker image creation isn't deterministic, you also realize that different people are producing different versions of images from the same Dockerfiles and that you have to spend extra bandwidth transferring the different versions around. This bites me all the time when I'm creating new images and am experimenting with the creation steps. I tend to bypass the fake caching mechanism (fake because the output isn't deterministic) and this really results in data explosion.

I understand why Docker images are neither deterministic nor minimal: making them so is a hard problem. I think Docker was right to prioritize solving distribution (it opens up many new possibilities). But I really wish some effort could be put into making images deterministic (and thus verifiable) and more minimal. I think it would make Docker an even more appealing platform, especially for the security conscious. (As an aside, I would absolutely love if we could ship a verifiable Firefox build, for example.)

These are hard problems. But they are solvable. Here's how I would do it.

First, let's tackle deterministic image creation. Despite computers and software being ideally deterministic, building software tends not to be, so deterministic image creation is a hard problem. Even tools like Puppet and Chef which claim to solve aspects of this problem don't do a very good job with determinism. Read my post on The Importance of Time on Machine Provisioning for more on the topic. But there are solutions. NixOS and the Nix package manager have the potential to be used as the basis of a deterministic image building platform. The high-level overview of Nix is that the inputs and contents of a package determine the package ID. If you know how Git or Mercurial get their commit SHA-1's, it's pretty much the same concept. In theory, two people on different machines start with the same environment and bootstrap the exact same packages, all from source. Gitian is a similar solution. Although I prefer Nix's content-based approach and how it goes about managing packages and environments. Nix feels so right as a base for deterministically building software. Anyway, yes, fully verifiable build environments are turtles all the way down (I recommend reading Tor's overview of the problem and their approach. However, Nix's approach addresses many of the turtles and silences most of the critics. I would absolutely love if more and more Docker images were the result of a deterministic build process like Nix. Perhaps you could define the full set of packages (with versions) that would be used. Let's call this the package manifest. You would then PGP sign and distribute your manifest. You could then have Nix step through all the dependencies, compiling everything from source. If PGP verification fails, compilation output changes, or extra files are needed, the build aborts or issues a warning. I have a feeling the security-minded community would go crazy over this. I know I would.

OK, so now you can use Nix to produce packages (and thus images) (more) deterministically. How do you make them minimal? Well, instead of just packaging the entire environment, I'd employ tools like makejail. The purpose of makejail is to create minimal chroot jail environments. These are very similar to Docker/LXC containers. In fact, you can often take a tarball of a chroot directory tree and convert it into a Docker container! With makejail, you define a configuration file saying among other things what binaries to run inside the jail. makejail will trace file I/O of that binary and copy over accessed files. The result is an execution environment that (hopefully) contains only what you need. Then, create an archive of that environment and pipe it into docker build to create a minimal Docker image.

In summary, Nix provides you with a reliable and verifiable build environment. Tools like makejail pair down the produced packages into something minimal, which you then turn into your Docker image. Regular people can still pull binary images, but they are much smaller and more in tune with Docker's principles of minimalism. The paranoid among us can produce the same bits from source (after verifying the inputs look credible and waiting through a few hours of compiling). Or, perhaps the individual files in the image could be signed and thus verified via trust somehow? The company deploying Docker can have peace of mind that disaster scenarios resulting in Docker image loss should not result in total loss of the image (just rebuild it exactly as it was before).

You'll note that my proposed solution does not involve Dockerfiles as they exist today. I just don't think Dockerfile's design of stackable layers of commands is the right model, at least for people who care about determinism and minimalism. You really want a recipe that knows how to create a set of relevant files and some metadata like what ports to expose, what command to run on container start, etc and turn that into your Docker image. I suppose you could accomplish this all inside Dockerfiles. But that's a pretty radical departure from how Dockerfiles work today. I'm not sure the two solutions are compatible. Something to think about.

I'm pretty sure of what it would take to add deterministic and verifiable building of minimal and more secure Docker images. And, if someone solved this problem, it could be applicable outside of Docker (again, Docker images are essentially chroot environments plus metadata). As I was putting the finishing touches on this article, I discovered nix-docker. It looks very promising! I hope the Docker community latches on to these ideas and makes deterministic, verifiable, and minimal images the default, not the exception.