Speed up backend Docker builds: defer src/ COPY to hosted, narrow scripts/ COPY, drop tests/

[LukeMainwaring]

Summary

• Move COPY src/ out of the shared app base into the hosted stage only — local already gets src/ from the ./backend/src:/app/src bind mount in docker-compose.yml, so editing source files no longer invalidates the local image cache.
• Replace COPY scripts/ /app/scripts/ with an explicit COPY scripts/run_local.sh scripts/run_hosted.sh ... — the other three *-docker.sh scripts are host-side wrappers around docker compose exec and don't belong in the image.
• Drop COPY tests/ from the local stage entirely — tests run on the host via uv run --directory backend pytest (per DEVELOPMENT.md) and CI doesn't pytest in Docker either.

Approach is modeled on LukeMainwaring/cortexdj#10, which solved the same problem in a sister project, with a few samplespace-specific extras.

Changes

• backend/Dockerfile: restructure the app/local/hosted stages so the only thing the shared base does is install system deps, copy the venv, and copy the two runtime entrypoint scripts. src/ now baked into hosted only.
• backend/.dockerignore: add .DS_Store, tests/, and scripts/*-docker.sh as defense-in-depth (and to shrink the build context tarball).

Test Plan

• docker compose build backend — initial build succeeds
• Immediate rebuild → all layers CACHED
• touch backend/src/samplespace/app.py && docker compose build backend → fully CACHED (the headline win — previously rebuilt the local image)
• touch backend/scripts/migrate-docker.sh && docker compose build backend → fully CACHED
• touch backend/tests/conftest.py && docker compose build backend → fully CACHED
• docker compose up -d backend → container boots, /app/src/samplespace/app.py visible via bind mount, /app/scripts/ contains only run_local.sh + run_hosted.sh, /app/tests does not exist
• docker build --target hosted backend/ — sanity-check the production stage still bundles src/ at /app/src (no bind mount in prod)
• One CI run on this branch to confirm nothing downstream depends on the old layout

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