Add Dockerfile.ci that extends the hexpm/elixir image with precompiled
EXLA NIF cached at ~/.cache/xla/. When mix test runs, EXLA's cached_make
finds the precompiled libexla.so and skips the OOM-prone g++ C++
compilation entirely.
- Dockerfile.ci: two-stage build — builder compiles EXLA into cache;
final stage copies the cache and installs test deps (cdo)
- .forgejo/workflows/build-ci-image.yaml: builds and pushes prop-ci
image on mix.exs/mix.lock/vendor changes + weekly cron
- .forgejo/workflows/build.yaml: test step uses prop-ci:latest instead
of raw hexpm/elixir image, removing apt-get + hex/rebar install steps
