fix(dockerignore): exclude local-only artifacts from build context (#10015)

The build context shipped to the daemon included several large
untracked directories the image never needs: saved image tarballs
(backend-images), locally-installed backends (local-backends), the
host-built binary (local-ai), the rust target/ build output, and
host node_modules/protoc/tests. This bloated the context to ~23GB.

Exclude them so only the sources the Dockerfile actually copies are
transferred. backend/rust sources stay tracked; only target/ is ignored.

Assisted-by: Claude:claude-opus-4-7 [Claude Code]

Signed-off-by: Richard Palethorpe <io@richiejp.com>

.dockerignore

@@ -30,3 +30,19 @@ backend/python/**/source
 # up compiled against whatever (likely older) commit the host had.
 backend/cpp/llama-cpp/llama.cpp
 backend/cpp/llama-cpp-*-build
+
+# Rust backend build output (sources are tracked; target/ is generated)
+backend/rust/*/target
+
+# Local-only artifacts that bloat the build context but the image never needs.
+# Saved image tarballs, locally-installed backends, the host-built binary, and
+# assorted tool/scratch dirs. None of these are git-tracked.
+backend-images
+local-backends
+local-ai
+.crush
+protoc
+tests
+
+# Installed via npm inside the build stage; no need to ship the host copy.
+**/node_modules