Harden the container: non-root, no build tools, read-only, stdout logging

The app ran as root, and the production image contained gcc, vim, wget
and gnupg. Code execution in the container would have allowed changing
the app code and fetching or compiling further tools.

- Multi-stage build: the wheels (psycopg2) are built in a build stage.
  The runtime image only contains libpq5, postgresql-client and
  wait-for-it (724 MB -> 298 MB).
- The app runs as the system user max (uid/gid 10001). The code is
  owned by root and precompiled, the user has no writable directory.
- Log to stdout only; docker collects it. The log file under /app/log
  was lost on every container recreation anyway.
- docker-compose: read-only filesystem (tmpfs for /tmp), no
  capabilities, no-new-privileges and log rotation (5 x 10 MB).
- Add a .dockerignore and set MAX_ORIGIN in docker-compose.yml.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,8 @@
+.git
+.devcontainer
+.vscode
+.claude
+docs
+log
+**/__pycache__
+**/*.pyc