documentation: adding gce-docker plugin to plugins.md
documentation: adding gce-docker plugin to plugins.md

Signed-off-by: Máximo Cuadros <6de7239924fbb83efa9bb5c97eb48a1df882d187@gmail.com>

thaJeztah/cli,e11137/cli,docker/cli,e11137/cli,allencloud/cli,dnephin/cli,redpanda/cli,cpuguy83/cli,cpuguy83/cli,ashfall/cli,riyazdf/cli,Zebrilee/cli,dnephin/cli,Zebrilee/cli,thaJeztah/cli,redpanda/cli,ashfall/cli,docker/cli,allencloud/cli,riyazdf/cli