build: add .dockerignore to trim the image build context

The Dockerfile installs its own deps inside each stage, so .venv, node_modules,
.git, tests, and the eNAV spreadsheets do not belong in the build context. Cuts
the context from ~379M to ~10M and speeds the s9pk image build.

.dockerignore

@@ -0,0 +1,18 @@
+# The image build installs its own deps (npm ci, pip install) inside the stages, so none of
+# these belong in the build context. Keeps the context small and the build fast.
+.git
+**/node_modules
+**/.venv
+**/__pycache__
+**/.pytest_cache
+**/.mypy_cache
+backend/tests
+backend/static
+deploy/*.s9pk
+deploy/javascript
+*.xlsx
+SECURITY-AUDIT.md
+*.md
+!README.md
+.DS_Store
+**/.DS_Store