Keep secrets and runtime data out of the Docker images

The build context is the repository root and there was no .dockerignore, so
'COPY backend/ ./' swept backend/venv (327 MB), backend/media (the actual
library, 224 MB), backend/logs, backend/staticfiles and backend/.env
(SECRET_KEY plus the database password) into the backend image: 1.43 GB per
architecture, including secrets headed for the registry. The frontend build
stage also copied the host's node_modules over the fresh install.

- Root .dockerignore excludes .git, virtualenvs, __pycache__, db.sqlite3,
  logs/staticfiles, .env files, media/, node_modules, dist and the deploy
  runtime state (data/, tailscale-state/).
- The backend Dockerfile now asserts the context is clean (.env, venv,
  media/library, db.sqlite3 all absent) before collectstatic, so a missing
  ignore file fails the build instead of leaking.
- Rebuilt: backend 1.43 GB -> 876 MB ('COPY backend/' is now 268 kB),
  frontend stays at 65 MB. Verified by booting the compose stack with the
  new image: migrations applied, /health ok, no .env or venv inside, and
  /app/media is the mounted (empty) volume; the scheduler runs too.
- Removed the stale local images that still contained the library and the
  dev .env.

.dockerignore

@@ -0,0 +1,35 @@
+# Build context is the repository root (see deploy/J621-Backend and
+# deploy/J621-Frontend). Keep the context small and, more importantly, keep
+# secrets and runtime data out of the images.
+
+# Version control / tooling
+.git
+.gitignore
+.dockerignore
+
+# Python: the dev virtualenv, caches, and anything generated at runtime
+backend/venv/
+**/__pycache__/
+**/*.py[cod]
+backend/db.sqlite3
+backend/logs/
+backend/staticfiles/
+
+# Secrets and media: the .env holds SECRET_KEY and DB passwords, media/ is
+# the actual library. The deploy composes mount these at runtime instead.
+backend/.env
+backend/.env.*
+backend/media/
+
+# Frontend build artefacts (npm ci installs fresh from the lockfile)
+frontend/node_modules/
+frontend/dist/
+
+# Deploy runtime state and env
+deploy/.env
+deploy/.env.*
+deploy/data/
+deploy/tailscale-state/
+
+# Misc
+*.log

deploy/J621-Backend

@@ -30,6 +30,14 @@ COPY backend/ ./
 COPY deploy/backend-entrypoint.sh /usr/local/bin/j621-entrypoint
 COPY deploy/scheduler-entrypoint.sh /usr/local/bin/j621-scheduler
 
+# Guard: fail the build if the context leaked secrets or runtime data
+# (.dockerignore excludes them — see the repository root).
+RUN test ! -e /app/.env \
+    && test ! -d /app/venv \
+    && test ! -d /app/media/library \
+    && test ! -e /app/db.sqlite3 \
+    && echo "build context clean"
+
 RUN chmod +x /usr/local/bin/j621-entrypoint /usr/local/bin/j621-scheduler \
     && mkdir -p /app/media /app/logs \
     && python manage.py collectstatic --noinput

deploy/README.md

@@ -114,6 +114,15 @@ ffmpeg for video thumbnails. The `GIT_HASH` build arg is baked into the
 backend image so the shell's version pill shows the commit (images have no
 `.git` directory); the push scripts pass it automatically.
 
+Both build from the repository root, which is filtered by `.dockerignore`:
+`backend/venv`, `backend/media`, `backend/logs`, `backend/staticfiles`,
+`backend/.env`, `frontend/node_modules`, `frontend/dist` and the deploy
+runtime state never enter the images — the database, media and logs come
+from the compose volumes and `deploy/.env` at runtime. The backend build
+also asserts that (`.env`, `venv`, `media/library`, `db.sqlite3` absent), so
+a missing ignore file fails the build instead of shipping secrets. Rebuild
+(and `docker image prune`) if you built before that guard existed.
+
 Push multi-arch images to the Gitea registry:
 
 ```bash