fix(dev): add grpc ext to docs container

[bshaffer]

Since the merging of #6454, the docs job has occasionally failed due to the grpc extension not being installed in the container. This was previously not necessary, as composer was not being run in the components, just in the dev directory (which does not require grpc)

Luckily this does not actually fail the job, only the storing of artifacts in placer.

cc @diptanshumittal

[Hectorhammett]

Looks straightforward to me 👍.

[bshaffer]

This should be updated automatically because of the following configuration, although this will be the first time we have tested it:

google-cloud-php/.kokoro/docs/common.cfg

Lines 31 to 41 in f7c3fd6

	# It will upload the docker image after successful builds.
	env_vars: {
	key: "TRAMPOLINE_IMAGE_UPLOAD"
	value: "true"
	}
	# It will always build the docker image.
	env_vars: {
	key: "TRAMPOLINE_DOCKERFILE"
	value: ".kokoro/docs/docker/Dockerfile"
	}