chore: install libgbm-dev to allow headless chrome e2e tests to run (#16695)

Without this lib, Chrome can’t set up its offscreen rendering buffers -
apparently.

I've validated this manually in my workspace.

Signed-off-by: Danny Kopping <danny@coder.com>

dogfood/contents/Dockerfile

@@ -160,6 +160,7 @@ RUN apt-get update --quiet && apt-get install --yes \
 	kubectl \
 	language-pack-en \
 	less \
+	libgbm-dev \
 	libssl-dev \
 	lsb-release \
 	man \