How to understand differences between docker and fly?

I ran this image on a local docker, and it worked fine: https://hub.docker.com/r/trafex/php-nginx

Running it unmodified in fly gave some errors the seem to be related to permissions when supervisord is starting up nginx and php-fpm:

2022-05-28T14:51:39Z app[2013add4] lhr [info]Preparing to run: `/usr/bin/supervisord -c /etc/supervisor/conf.d/supervisord.conf` as nobody
2022-05-28T14:51:39Z app[2013add4] lhr [info]2022/05/28 14:51:39 listening on [fdaa:0:6561:a7b:aa9c:2013:add4:2]:22 (DNS: [fdaa::3]:53)
2022-05-28T14:51:40Z app[2013add4] lhr [info]2022-05-28 14:51:40,320 INFO supervisord started with pid 515
2022-05-28T14:51:41Z app[2013add4] lhr [info]2022-05-28 14:51:41,322 INFO spawnerr: unknown error making dispatchers for 'nginx': EACCES
2022-05-28T14:51:41Z app[2013add4] lhr [info]2022-05-28 14:51:41,322 INFO spawnerr: unknown error making dispatchers for 'php-fpm': EACCES

The Dockerfile had USER nobody and removing this and fixing things up made it run in fly.

How can I understand what the differences between docker and fly are here?

Fly transmogrifies runc containers into Firecracker VMs supervised by its own init which in addition to other things is fussy at letting any other process read from stdout.

root is the safest bet to have everything running smoothly, apparently, even if it needn’t be the case.

The specific issue you’re seeing has been discussed a number of times on this very forum. Here’s a recent discussion: Allow non-root users to write to stdout/stderr - #13 by ignoramous

As you (and others) found out Dockerfiles on Fly aren’t the same as Dockerfiles everywhere else. For Fly, Dockerfiles are a convinient way to hook into the existing ecosystem: Early look: "fly launch" That said, it works almost all of the time, until it doesn’t.

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