We recently adopted the (previously community-developed) Docker image for Zulip, and I've gotten a bunch of experience as part of this process. We didn't have a lot of choice as to whether to use Docker -- as an open source web application, installation options are driven by what users are asking for, and in the container world, that's a Docker image.

To be honest, maintaining a Docker image is kind of a pain, even for something like Zulip where our Debian/Ubuntu install process is super clean and reliable. Docker requires you to run your application's installer in a totally crippled environment (E.g. no unicode locales are installed, so using pip to build Python packages doesn't work until you fix that) to generate a working Dockerfile. But the real pain is the fact that Docker users expect to configure everything via environment variables in e.g. a docker-compose.yml or similar Kubernetes configuration file.

And that results in just about every Docker image for a reusable web application having a giant docker-entrypoint.sh script of semi-duplicated code (ours is currently ~600LOC; those for other popular projects can easily be 2-3X that size) that does a bunch of stuff, including pass what is structurally an array through a shell environment variable and back into our Python-format settings file, which is just a mess.

Since that shell script can have bugs, this is a big maintenance (e.g. right now probably a third of user reports of issues installing a new Zulip server are Docker-specific issues, even though Docker installations are a small fraction of total installations).

Further, a lot of third-party images (e.g. for postgres or redis or RabbitMQ) end up having issues where they have an environment variable for configuring something (E.g. a postgres password), but if you change that variable after the postgres image is first booted, nothing happens, which can be confusing for users (who often assume they can just boot their image with whatever password and clean up the security stuff later).

So, while the Docker/containers ecosystem has a lot of promise, I also feel it has a long way to go before providing a Docker image isn't a significant burden on projects, even ones like us that already have a battle-tested installation process.

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