Read-only Docker Containers

There are lots of good reasons for and articles recommending running Docker containers read-only, but what I have a difficult time finding are descriptions of how to do this for many popular images. Some software needs to write to a few important and predictable locations. It surprises me how often image providers neglect to offer instructions or details required to run their image this way.

Even setting aside read-only containers, counting on writing to the writable layer just feels wrong. Per the documentation, for the writable layer, both read and write speeds are lower because of the copy-on-write/overlay process through the storage driver. In my experience, docker diff output means I haven’t taken the time to configure my volume declarations, either through tmpfs mounts, volumes, or bind mounts.

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Distributing CLI Tools via Docker

Throughout my career, I’ve seen a couple recurring patterns related to the tools I write: I write a lot of small CLI tools and I like to share them with my coworkers (and whenever possible, the rest of the world).

This has led to several iterations of solving the problem How do I make this tool easy to run? since I don’t want to burden people with understanding the intricacies of all my tools’ dependencies. These tend to be Ruby, some number of gems, and possibly some other common unix utilities. The solutions I’ve come up with have included a lengthy README with detailed instructions, Bundler with Rake tasks to do all the heavy lifting for non-Ruby things, fpm, and even “curl bash piping” (yes, I’m horrible).

Recently I decided to use Docker to solve this problem, since I’m using it so much anyway. Using Docker has some huge benefits for sharing applications of all types: the dependencies list gets whittled down to just Docker, things work on more platforms, testing gets simpler, and it is the new hotness which makes people say “whoa” and that’s fun. That said, the downsides can be frustrating: working with files on your machine gets messy, more typing with the extra Docker-related preamble, things are less straightforward and clear, simple mistakes can lead to lots of images and containers to clean up, and the executable gets significantly larger (since the Docker image is a whole, albeit lightweight, OS userland to run the app). After weighing these pros and cons, I’ve found that telling a coworker to docker pull registry.url/my/app and run it with --help is so much more convenient than the alternatives.

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