Previously, `astro dev pytest` created an ephemeral container and only
copied `dags/` and `.astro/` into it via `docker cp`. This meant
`tests/`, `plugins/`, and `include/` were only available if baked into
the Docker image with COPY, unlike `astro dev start` which bind-mounts
all four directories via docker-compose.

Switch to `-v` bind mounts on `docker create` for dags/, tests/,
plugins/, and include/ so that host files are always visible without
needing COPY in the Dockerfile. The `.astro/` folder continues to use
`docker cp` due to inconsistent `.dockerignore` behavior across
platforms.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>