I'm trying to do the exact same thing using docker images, and it's been really rough. I have each one linked to my redis and postgres containers (Which are both running correctly and have been tested with other services), and am using a configuration that should work correctly.
So far, the issue on any containers that are using the most recent version of PixelFed seems to be that when using a postgres container it claims that the driver could not be found. However, php7-pgsql is installed according to the built-in alpine linux package manager (which I'm starting to despise). I've tried it a bit with other less-recent containers and have had other varied issues, but with the most recent containers (Which are likely all based on the same container), that seems to be the issue.
For reference, this seems to be the origin of the containers that use the latest Pixelfed: