Fix CI for Gitea Actions: remove volume mount, run all commands in container

The docker-compose.ci.yml mounted '.:/app' which works on GitHub Actions
(where the workspace is the repo root) but not on Gitea Actions (where
the workspace path differs). The built Docker image already has all files
from COPY . ./ so the volume mount is unnecessary for CI.

Also moved yarn install inside the container and run full test suite.

docker-compose.ci.yml

@@ -5,11 +5,6 @@ services:
       dockerfile: ./docker/dev.Dockerfile
     environment:
       - MIX_ENV=test
-    volumes:
-      - '.:/app'
-      # These lines ensure the deps can be saved as build artifacts for caching
-      - '/app/deps'
-      - '/app/_build'
     ports:
       - '4008:4008'
     command: tail -F /dev/null