add plastid docker

src/.docker_modules/plastid/0.5.1/Dockerfile

+FROM quay.io/biocontainers/plastid:0.5.1--py36h955c1b8_2

src/.docker_modules/plastid/0.5.1/docker_init.sh

+#!/bin/sh
+docker pull lbmc/plastid:0.5.1
+docker build src/.docker_modules/plastid/0.5.1 -t 'lbmc/plastid:0.5.1'
+docker push lbmc/plastid:0.5.1