So in the official docker-compose.yml lines that define where/how to get the image for that application.

For example:

https://github.com/LemmyNet/lemmy/blob/c9e9ff46faa40e2642343effb117693bfa525c5f/docker/docker-compose.yml#L41-L43

build:
      context: ../
      dockerfile: docker/Dockerfile

This tells docker to look for a file called docker/Dockerfile in the parent directory. This means that when you go to call docker compose up -d it will build an image from source using that Dockerfile. For the Pi we don't want this (at least as of 0.17.x; I haven't tested 0.18.0 yet).

Instead we want to use a pre-built image. To do that we need to go to docker hub, specifically: https://hub.docker.com/r/dessalines/lemmy/tags and find the latest tag that matches the architecture of the system we're building on. I assume you're on a Pi4 running a 64bit so, so that gives us 0.17.3-linux-arm64. After you've got that tag we just need to replace those 3 lines above with:

image: dessalines/lemmy:0.17.3-linux-arm64

Now when we go to call docker compose up -d it will pull down that prebuilt image instead of building for source. Btw, you'll want to do the same for the lemmy-ui service.

P.S. I don't have much experience using Ansible, so I can't help here. I normally just SSH directly into the Pi and do everything there.