bin: add `docker-gc` helper

Signed-off-by: Alejandro Mery <amery@geeks.cl>

files/bin/docker-gc

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+set -x
+
+docker ps -a -q -f status=exited | xargs -r docker rm -v
+docker images -f "dangling=true" -q | xargs -r docker rmi