Add a .dockerignore (#1591)

tests/README.md has developers clone Home Assistant core into ./core, but
Docker does not read .gitignore, so `COPY . /app/` shipped that ~300MB
checkout into the build context and into the image on every build.

It also changed what the build did. With /app/core already present as a
real directory, `ln -s /core /app/core` linked *inside* it — leaving a
stray /app/core/core -> /core — and scripts/setup-dependencies then
installed from the copied host checkout rather than the image's own
pinned clone.

Excluding core/ (plus local virtualenvs, VCS state and caches) takes the
build context from 412MB to 4.6MB and the image from 2.34GB to 2.1GB, and
makes a build with a local ./core behave like a clean one: /app/core is
the intended symlink to /core.

This does remove an accident. An image built while a local ./core existed
happened to run without `-v $(pwd):/app`, because the copied checkout
carried relative symlinks that still resolved inside /app. A clean-checkout
build never had that property — there the symlinks setup-symlinks writes
into /core dangle — and tests/README.md requires the mount either way.

479 passed, unchanged.

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,28 @@
+# The Home Assistant core checkout. tests/README.md has you clone it to ./core,
+# but the Dockerfile clones its own copy to /core and links /app/core to it.
+# Without this entry `COPY . /app/` ships ~300MB into every build and leaves
+# /app/core as a real directory, so `ln -s /core /app/core` links *inside* it
+# rather than creating the intended symlink.
+core/
+
+# Local virtualenvs
+.venv/
+venv/
+env/
+ENV/
+
+# Not used by the build
+.git/
+.vscode/
+.idea/
+
+# Caches and test output
+__pycache__/
+*.py[cod]
+.pytest_cache/
+.ruff_cache/
+.mypy_cache/
+htmlcov/
+.coverage
+.coverage.*
+coverage.xml