Add docker group to Buildkite docker plugin config (#2699)

Fix [this
error](https://buildkite.com/bazel/bazel-bazel-github-presubmit/builds/34290#019f1cb3-0d36-48f5-8f69-f4bba4be27cc/L906),
caused by the [security hardening
commit](https://github.com/bazelbuild/continuous-integration/commit/c1c1db8dbd0cff3b8d00d79530329878082d4f89),
which restricted the host's Docker socket permissions to 0660 (allowing
access only to root and members of the docker group).

Because the Buildkite agent runs CI build steps inside Docker containers
under a custom user ID, the containerized processes lacked permission to
communicate with the host's hardened docker.sock. Adding "docker" group
to our Buildkite Docker plugin configuration resolves this by granting
the containers the necessary group membership to access the socket and
execute Docker commands successfully.