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>